Multiemployer Pension Plans (Details) $ in Millions		12 Months Ended
		Dec. 31, 2017 USD ($) store agreements	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Multiemployer Plans [Line Items]
Total contributions	[1]	$ 1.6	$ 1.5	$ 1.4
Automotive Industries Pension Plan [Member]
Multiemployer Plans [Line Items]
EIN		941133245
Multiemployer plan number		001
Pension protection act zone status		Red	Red
Total contributions	[1]	$ 1.3	$ 1.1	1.0
Surcharge imposed		Yes
Number of collective-bargaining arrangements that require contributions to the Plan | agreements		2
Other funds [Member]
Multiemployer Plans [Line Items]
Total contributions	[1]	$ 0.3	$ 0.4	$ 0.4
Dealerships [Member]
Multiemployer Plans [Line Items]
Number of stores | store		253
Dealerships [Member] | Multiemployer Plans, Pension [Member]
Multiemployer Plans [Line Items]
Number of stores | store		5

[1] Our stores were not listed in the Automotive Industries Pension Plan’s Form 5500 as providing more than 5% of the total contributions for the plan years ended December 31, 2016 or 2015.