FLOOR PLAN NOTES PAYABLE—NON-TRADE (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Oct. 16, 2014
New Vehicle Revolving Floor Plan Facility
Debt Instrument [Line Items]
Commitment fee percentage	0.15%
New Vehicle Revolving Floor Plan Facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.25%
New Vehicle Revolving Floor Plan Facility | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	0.25%
Used Vehicle Revolving Floor Plan Facility
Debt Instrument [Line Items]
Commitment fee percentage	0.15%
Used Vehicle Revolving Floor Plan Facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.50%
Used Vehicle Revolving Floor Plan Facility | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Senior Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 325,000,000.0
Senior Credit Facility | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Senior Credit Facility | Federal Funds
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Bank of America, N.A. | Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 250,000,000.0
Bank of America, N.A. | Standby Letters of Credit
Debt Instrument [Line Items]
Face amount	50,000,000.0	$ 50,000,000
Bank of America, N.A. | New Vehicle Revolving Floor Plan Facility
Debt Instrument [Line Items]
Current borrowing capacity	60,000,000
Maximum borrowing capacity	900,000,000.0
Remaining borrowing capacity	190,000,000
Bank of America, N.A. | Used Vehicle Revolving Floor Plan Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	150,000,000.0
Unused borrowing capacity	$ 78,600,000