Commitments and Contingencies - Summary of Lease Exit Accruals (Parenthetical) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Component of lease exit expense in interest expense, other, net	$ 0.1
Component of lease exit expense in selling, general and administrative expenses	0.9
Component of lease exit expense in income (loss) from operations and the sale of dealerships	1.2
Component of lease exit payments in selling, general and administrative expenses	1.1
Component of lease exit payments in income (loss) from operations and the sale of dealerships	$ 2.5