Employee Benefit Plans (Tables)
   | 
12 Months Ended | 
Dec. 31, 2017  | 
| Compensation And Retirement Disclosure [Abstract] | 
 
 | 
| Status of Stock Options Related to Stock Plans | 
A summary of the status of the stock options related to the Stock Plans is presented below:  
  
| 
   
 | 
   
 | 
 Options 
Outstanding 
 | 
   
 | 
   
 | 
 Exercise Price 
Per Share 
(Low - High) 
 | 
   
 | 
 Weighted 
Average 
Exercise 
Price 
Per Share 
 | 
   
 | 
   
 | 
 Weighted 
Average 
Remaining 
Contractual 
Term 
 | 
   
 | 
   
 | 
 Aggregate 
Intrinsic 
Value 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 (In thousands, except per share data, term in years) 
 | 
   
 | 
 
| 
 Balance at December 31, 2016 
 | 
   
 | 
   
 | 
 439 
 | 
   
 | 
   
 | 
 $ 
 | 
 1.81 
 | 
   
 | 
 - 
 | 
  30.07 
 | 
   
 | 
 $ 
 | 
 13.42 
 | 
   
 | 
   
 | 
   
 | 
 1.4 
 | 
   
 | 
   
 | 
 $ 
 | 
 5,327 
 | 
   
 | 
 
| 
 Exercised 
 | 
   
 | 
   
 | 
 (25 
 | 
 ) 
 | 
   
 | 
 $ 
 | 
 1.81 
 | 
   
 | 
 - 
 | 
  1.81 
 | 
   
 | 
 $ 
 | 
 1.81 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 Forfeited 
 | 
   
 | 
   
 | 
 (186 
 | 
 ) 
 | 
   
 | 
 $ 
 | 
 28.04 
 | 
   
 | 
 - 
 | 
  30.07 
 | 
   
 | 
 $ 
 | 
 29.14 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 Balance at December 31, 2017 
 | 
   
 | 
   
 | 
 228 
 | 
   
 | 
   
 | 
 $ 
 | 
 1.81 
 | 
   
 | 
 - 
 | 
  1.81 
 | 
   
 | 
 $ 
 | 
 1.81 
 | 
   
 | 
   
 | 
   
 | 
 1.3 
 | 
   
 | 
   
 | 
 $ 
 | 
 3,787 
 | 
   
 | 
 
| 
 Exercisable 
 | 
   
 | 
   
 | 
 228 
 | 
   
 | 
   
 | 
 $ 
 | 
 1.81 
 | 
   
 | 
 - 
 | 
  1.81 
 | 
   
 | 
 $ 
 | 
 1.81 
 | 
   
 | 
   
 | 
   
 | 
 1.3 
 | 
   
 | 
   
 | 
 $ 
 | 
 3,787 
 | 
   
 | 
 
 
 
  
| 
   
 | 
   
 | 
 Year Ended December 31, 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 2017 
 | 
   
 | 
   
 | 
 2016 
 | 
   
 | 
   
 | 
 2015 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 (In thousands) 
 | 
   
 | 
 
| 
 Intrinsic value of stock options exercised 
 | 
   
 | 
 $ 
 | 
 425 
 | 
   
 | 
   
 | 
 $ 
 | 
 250 
 | 
   
 | 
   
 | 
 $ 
 | 
 2,511 
 | 
   
 | 
 
 
 
  
 
 | 
| Status of Non-Vested Restricted Stock and Restricted Stock Unit Grants Related to Stock Plans | 
A summary of the status of non-vested restricted stock award and restricted stock unit grants related to the Stock Plans is presented below:  
  
| 
   
 | 
   
 | 
 Non-Vested 
Restricted 
Stock Awards 
and Restricted 
Stock Units 
 | 
   
 | 
   
 | 
 Weighted 
Average 
Grant Date 
Fair Value 
per Share 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 (In thousands, except per share data) 
 | 
   
 | 
 
| 
 Balance at December 31, 2016 
 | 
   
 | 
   
 | 
 2,180 
 | 
   
 | 
   
 | 
 $ 
 | 
 20.86 
 | 
   
 | 
 
| 
 Granted 
 | 
   
 | 
   
 | 
 600 
 | 
   
 | 
   
 | 
 $ 
 | 
 22.88 
 | 
   
 | 
 
| 
 Forfeited 
 | 
   
 | 
   
 | 
 (131 
 | 
 ) 
 | 
   
 | 
 $ 
 | 
 16.59 
 | 
   
 | 
 
| 
 Vested 
 | 
   
 | 
   
 | 
 (450 
 | 
 ) 
 | 
   
 | 
 $ 
 | 
 20.42 
 | 
   
 | 
 
| 
 Balance at December 31, 2017 
 | 
   
 | 
   
 | 
 2,199 
 | 
   
 | 
   
 | 
 $ 
 | 
 21.76 
 | 
   
 | 
 
 
 
  
 
 | 
| Status of Supplemental Executive Retirement Plan | 
The following table sets forth the status of the SERP:   
  
| 
   
 | 
   
 | 
 Year Ended December 31, 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 2017 
 | 
   
 | 
   
 | 
 2016 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 (In thousands) 
 | 
   
 | 
 
| 
 Change in projected benefit obligation: 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 Obligation at January 1, 
 | 
   
 | 
 $ 
 | 
 11,233 
 | 
   
 | 
   
 | 
 $ 
 | 
 9,234 
 | 
   
 | 
 
| 
 Service cost 
 | 
   
 | 
   
 | 
 1,711 
 | 
   
 | 
   
 | 
   
 | 
 1,590 
 | 
   
 | 
 
| 
 Interest cost 
 | 
   
 | 
   
 | 
 448 
 | 
   
 | 
   
 | 
   
 | 
 383 
 | 
   
 | 
 
| 
 Actuarial loss (gain) 
 | 
   
 | 
   
 | 
 429 
 | 
   
 | 
   
 | 
   
 | 
 295 
 | 
   
 | 
 
| 
 Amendments/settlements/curtailments loss (gain) 
 | 
   
 | 
   
 | 
 - 
 | 
   
 | 
   
 | 
   
 | 
 - 
 | 
   
 | 
 
| 
 Benefits paid 
 | 
   
 | 
   
 | 
 (265 
 | 
 ) 
 | 
   
 | 
   
 | 
 (269 
 | 
 ) 
 | 
 
| 
 Obligation at December 31, (1) 
 | 
   
 | 
 $ 
 | 
 13,556 
 | 
   
 | 
   
 | 
 $ 
 | 
 11,233 
 | 
   
 | 
 
| 
 Accumulated benefit obligation 
 | 
   
 | 
 $ 
 | 
 10,204 
 | 
   
 | 
   
 | 
 $ 
 | 
 8,557 
 | 
   
 | 
 
 
 
  
| 
 (1) 
 | 
 Approximately $13.3 million is included in other long-term liabilities and approximately $0.3 million is included in other accrued liabilities in the accompanying consolidated balance sheets.  
 | 
 
 
 
  
| 
   
 | 
   
 | 
 Year Ended December 31, 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 2017 
 | 
   
 | 
   
 | 
 2016 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 (In thousands) 
 | 
   
 | 
 
| 
 Change in fair value of plan assets: 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 Plan assets at January 1,  
 | 
   
 | 
 $ 
 | 
 - 
 | 
   
 | 
   
 | 
 $ 
 | 
 - 
 | 
   
 | 
 
| 
 Actual return on plan assets 
 | 
   
 | 
   
 | 
 - 
 | 
   
 | 
   
 | 
   
 | 
 - 
 | 
   
 | 
 
| 
 Employer contributions 
 | 
   
 | 
   
 | 
 265 
 | 
   
 | 
   
 | 
   
 | 
 269 
 | 
   
 | 
 
| 
 Benefits paid 
 | 
   
 | 
   
 | 
 (265 
 | 
 ) 
 | 
   
 | 
   
 | 
 (269 
 | 
 ) 
 | 
 
| 
 Plan assets at December 31, 
 | 
   
 | 
   
 | 
 - 
 | 
   
 | 
   
 | 
   
 | 
 - 
 | 
   
 | 
 
| 
 Funded status recognized 
 | 
   
 | 
 $ 
 | 
 (13,556 
 | 
 ) 
 | 
   
 | 
 $ 
 | 
 (11,233 
 | 
 ) 
 | 
 
 
 
  
 
 | 
| Cost Components of Supplemental Executive Retirement Plan | 
The following table provides the cost components of the SERP:  
  
| 
   
 | 
   
 | 
 Year Ended December 31, 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 2017 
 | 
   
 | 
   
 | 
 2016 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 (In thousands) 
 | 
   
 | 
 
| 
 Service cost 
 | 
   
 | 
 $ 
 | 
 1,711 
 | 
   
 | 
   
 | 
 $ 
 | 
 1,590 
 | 
   
 | 
 
| 
 Interest cost 
 | 
   
 | 
   
 | 
 448 
 | 
   
 | 
   
 | 
   
 | 
 383 
 | 
   
 | 
 
| 
 Net pension expense (benefit) 
 | 
   
 | 
 $ 
 | 
 2,159 
 | 
   
 | 
   
 | 
 $ 
 | 
 1,973 
 | 
   
 | 
 
 
 
  
 
 | 
| Weighted Average Assumptions Used to Determine Benefit Obligation and Net Periodic Benefit Costs | 
The weighted average assumptions used to determine the benefit obligation and net periodic benefit costs consist of:   
  
| 
   
 | 
   
 | 
 As of December 31, 
 | 
   
 | 
 
| 
   
 | 
   
 | 
 2017 
 | 
   
 | 
   
 | 
 2016 
 | 
   
 | 
 
| 
 Discount rate 
 | 
   
 | 
   
 | 
 3.50 
 | 
 % 
 | 
   
 | 
   
 | 
 4.04 
 | 
 % 
 | 
 
| 
 Rate of compensation increase 
 | 
   
 | 
   
 | 
 3.00 
 | 
 % 
 | 
   
 | 
   
 | 
 3.00 
 | 
 % 
 | 
 
 
 
  
 
 | 
| Estimated Future Benefit Payments | 
The estimated future benefit payments expected to be paid for each of the next five years and the sum of the payments expected for the next five years thereafter are:  
  
| 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
   
 | 
   
 | 
 Estimated Future Benefit Payments 
 | 
   
 | 
 
| 
 Year Ending December 31, 
 | 
   
 | 
 (In thousands) 
 | 
   
 | 
 
| 
 2018 
 | 
   
 | 
 $ 
 | 
 265 
 | 
   
 | 
 
| 
 2019 
 | 
   
 | 
 $ 
 | 
 265 
 | 
   
 | 
 
| 
 2020 
 | 
   
 | 
 $ 
 | 
 363 
 | 
   
 | 
 
| 
 2021 
 | 
   
 | 
 $ 
 | 
 363 
 | 
   
 | 
 
| 
 2022 
 | 
   
 | 
 $ 
 | 
 363 
 | 
   
 | 
 
| 
 2023 - 2027 
 | 
   
 | 
 $ 
 | 
 1,968 
 | 
   
 | 
 
 
 
  
 
 | 
| Schedule of Multiemployer Pension Plans Affecting Period-to-Period Comparability of Contributions | 
The number of employees covered by the AI Pension Plan increased 2.6% from December 31, 2015 to December 31, 2016 and increased 0.5% from December 31, 2016 to December 31, 2017, affecting the period-to-period comparability of the contributions for 2017, 2016 and 2015. 
 
| 
   
 | 
   
 | 
   
 | 
   
 | 
 Pension 
Protection 
Act Zone 
Status 
 | 
   
 | 
 FIP/RP Status 
 | 
   
 | 
 Sonic Contributions 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 Collective 
Bargaining 
 | 
 
| 
 Pension 
 | 
   
 | 
 EIN/Pension 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 Pending / 
 | 
   
 | 
 Year Ended December 31, 
 | 
   
 | 
   
 | 
 Surcharge 
 | 
   
 | 
 Agreement 
 | 
 
| 
 Fund 
 | 
   
 | 
 Plan Number 
 | 
   
 | 
 2017 
 | 
   
 | 
 2016 
 | 
   
 | 
 Implemented 
 | 
   
 | 
 2017 
 | 
   
 | 
   
 | 
 2016 
 | 
   
 | 
   
 | 
 2015 
 | 
   
 | 
   
 | 
 Imposed 
 | 
   
 | 
 Expiration Date (1) 
 | 
 
| 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 (In thousands) 
 | 
   
 | 
   
 | 
   
 | 
   
 | 
   
 | 
 
| 
 AI Pension Plan 
 | 
   
 | 
 94-1133245 
 | 
   
 | 
 Red 
 | 
   
 | 
 Red 
 | 
   
 | 
 RP Implemented 
 | 
   
 | 
 $ 
 | 
 171 
 | 
   
 | 
   
 | 
 $ 
 | 
 150 
 | 
   
 | 
   
 | 
 $ 
 | 
 140 
 | 
   
 | 
   
 | 
 Yes 
 | 
   
 | 
 Between 
May 21, 2018 
and November 15, 2018 
 | 
 
 
 
  
| 
 (1) 
 | 
 Collective bargaining agreement expiration dates vary by union and dealership. Dates shown represent the range of the earliest and latest stated expirations for Sonic’s union employees, noting certain of Sonic’s collective bargaining agreements are expired as of December 31, 2017 and are currently under negotiation. 
 | 
 
 
 
 
 |