Note 7 - Floor Plan Notes Payable and Lines of Credit (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility, Maximum Borrowing Capacity	$ 17,500,000
Floor Plan Notes Payable	1,023,019,000	$ 778,561,000
Long-term Line of Credit, Total	0
Line of Credit Amount Pledged to Secure Letters of Credit	11,600,000
Line of Credit Facility, Remaining Borrowing Capacity	$ 5,900,000
Floor Plan Credit Agreement [Member]
Debt, Weighted Average Interest Rate	1.90%	1.40%
BMO Harris Bank N.A. [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 875,000,000
Debt Instrument, Working Capital Fee Percentage	0.16%
Floor Plan Notes Payable	$ 798,400,000
BMO Harris Bank N.A. [Member] | IfTermination Occurs after July 1, 2018 and Prior to June 30, 2019 [Member]
Debt Instrument, Prepayment Processing Fee Amount	$ 500,000
BMO Harris Bank N.A. [Member] | Floor Plan Credit Agreement [Member]
Debt Instrument, Interest Rate, Effective Percentage	4.26%
BMO Harris Bank N.A. [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate	1.51%
Ford Motor Financing Agreement [Member]
Floor Plan Notes Payable	$ 139,000,000
Debt Instrument, Variable Rate, Minimum Reference Rate	7.00%
Ford Motor Financing Agreement [Member] | Prime Rate [Member]
Debt Instrument, Basis Spread on Variable Rate	1.50%
Revolving Credit Facility [Member] | BMO Harris Bank N.A. [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 100,000,000
Long-term Line of Credit, Total	0
Letter of Credit [Member] | BMO Harris Bank N.A. [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 20,000,000