Summary of Significant Accounting Policies - Summary of Lease Exit Accruals (Parenthetical) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Accounting Policies [Abstract]
Component of lease exit expense in interest expense, other, net and expense	$ 0.1
Component of lease exit expense in selling, general and administrative expenses	0.8
Component of lease exit expense in income (loss) from operations and the sale of dealerships before tax	1.1
Component of lease exit payments in selling, general and administrative expenses	0.7
Component of lease exit payments in income (loss) from operations and the sale of dealerships before tax	$ 2.0