Commitments and Contingencies - Additional Information (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Option	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Contingencies And Commitments [Line Items]
Lease exit expense	$ 1.0	$ 2.2
Interest Expense		0.6
Lease exit accrued cost		$ 1.6
Operating lease term for dealership facilities, minimum		15 years
Operating lease term for dealership facilities, maximum		20 years
Operating lease number of renewal options, minimum | Option		1
Operating lease number of renewal options, maximum | Option		2
Operating lease period of renewal options, minimum		5 years
Operating lease period of renewal options, maximum		10 years
Percentage of lease facility based on capitalization rates		10.00%
Lease expense for continuing operation		$ 100.6	$ 94.6	$ 98.2
Lease expense from discontinuing operation net of lease exit accrual adjustments		1.3	0.9	1.4
Leases, net contingent rent benefit related to decrease in interest rates from continuing operation		1.7	1.8	2.0
Leases net contingent rent benefit related to decrease in interest rates from discontinuing operation		0.1	0.1	$ 0.1
Obligations under subleases, if subleases do not perform		47.7
Maximum exposure associated with general indemnifications		$ 5.0
General indemnifications expiration period		1 year
Contingent liability reserve balance after reduction		$ 2.8
Other Accrued Liabilities [Member]
Contingencies And Commitments [Line Items]
Amount reserved for pending proceedings		3.0	0.3
Other Long-Term Liabilities [Member]
Contingencies And Commitments [Line Items]
Amount reserved for pending proceedings		$ 0.2	$ 0.2
Dealership [Member]
Contingencies And Commitments [Line Items]
Joint venture ownership percentage		50.00%