Sonic Automotive, Inc. Annual report pursuant to Section 13 and 15(d)

Income Taxes - Summary of Changes in Liability Related to Unrecognized Tax Benefits (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Unrecognized Tax Benefits [Line Items]
Accrued interest and penalties	$ 1.2	$ 1.1	$ 2.4	$ 4.9
State and Local Jurisdiction [Member]
Schedule Of Unrecognized Tax Benefits [Line Items]
Net operating losses	$ 0.8	$ 1.0	$ 1.3