Summary of Significant Accounting Policies - Summary of Operating Lease Exit Accruals (Parenthetical) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($)
Accounting Policies [Abstract]
Components of lease exit expense in interest expense, other, net	$ 0.1
Component of lease exit expense in selling, general and administrative expenses	0.2
Component of lease exit expense in income (loss) from operations and the sale of dealerships before tax	1.2
Component of lease exit payments in selling, general and administrative expenses	0.5
Component of lease exit payments in income (loss) from operations and the sale of dealerships before tax	$ 4.2