Annual report pursuant to Section 13 and 15(d)

Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail)

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Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
Jan. 01, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dealerships
State
Brand
Collision
Store
Segment
Franchise
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Description Of Business And Summary Of Significant Accounting Policies [Line Items]        
Number of new vehicle dealerships | Dealerships   114    
Number of states | State   13    
Number of different brands of cars and light trucks | Brand   23    
Number of collision repair centers | Collision   18    
Number of pre-owned stores | Store   9    
Number of operating segments | Segment   2    
Book overdraft position   $ 6,900,000 $ 8,000,000  
Revenue allowances for commission reserves   20,900,000 19,200,000  
Amount recognized for floor plan assistance   $ 45,300,000 45,000,000 $ 42,100,000
Term for funding of finance contracts   10 days    
Contracts in transit included in receivables, net   $ 267,600,000 236,400,000  
Number of reporting units | Franchise   2    
Goodwill   $ 525,780,000 472,437,000 471,493,000
Insurance reserves   22,000,000 22,700,000  
Concentrations of credit risk with respect to receivables are limited primarily to receivables from automobile manufacturers   102,500,000 92,800,000  
Deposits   3,000,000 10,000,000  
Reduction in interest expense   500,000 600,000 1,500,000
Advertising expense   61,600,000 61,700,000 61,600,000
Cooperative manufacturer credits advertising expenses   $ 26,000,000 26,200,000 24,200,000
Number of reportable Segment | Segment   2    
Financial Institutions [Member]        
Description Of Business And Summary Of Significant Accounting Policies [Line Items]        
Concentrations of credit risk with respect to receivables are limited primarily to receivables from financial institutions   $ 299,600,000 265,300,000  
Franchise Assets [Member]        
Description Of Business And Summary Of Significant Accounting Policies [Line Items]        
Franchise asset impairment charge   3,600,000    
Continuing Operations [Member] | Franchise Assets [Member]        
Description Of Business And Summary Of Significant Accounting Policies [Line Items]        
Franchise asset impairment charge   $ 3,600,000 $ 0 $ 3,300,000
Number of franchises resulted in asset impairment charge | Franchise   4    
Accounting Standards Update (“ASU”) 2014-09 [Member] | Subsequent Event [Member]        
Description Of Business And Summary Of Significant Accounting Policies [Line Items]        
Cumulative effect adjustment to increase retained earnings, net $ 3,000,000      
Dealership [Member]        
Description Of Business And Summary Of Significant Accounting Policies [Line Items]        
Percentage of dealership that is accounted for under the equity method   50.00%    
Franchised Dealerships Segment [Member]        
Description Of Business And Summary Of Significant Accounting Policies [Line Items]        
Number of stores | Store   103    
Goodwill   $ 465,800,000    
Pre-Owned Stores Segment [Member]        
Description Of Business And Summary Of Significant Accounting Policies [Line Items]        
Number of stores | Store   9    
Goodwill   $ 60,000,000