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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% |  |

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