Interest Rate Swaps (Details) (Interest Rate Swap Agreements, USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Fair Value, Inputs, Level 2	Dec. 31, 2012 Fair Value, Inputs, Level 2	Jun. 30, 2013 Floating Rate Floor Plan Debt	Jun. 30, 2013 Floating Rate Floor Plan Debt	Jun. 30, 2013 Floating Rate Floor Plan Debt LIBOR portion	Jun. 30, 2013 Floating Rate Floor Plan Debt Fixed rate portion
Derivative
Portion of floating rate floor plan debt fixed by swap agreements							$ 300,000	$ 100,000
Interest rate swap, fixed (as a percent)							2.135%	1.55%
Estimated liabilities of swaps designated as hedging instruments, fair value			10,743	14,337
Hedge ineffectiveness recorded	$ 0	$ 0
Increase in the weighted average interest rate on floor plan borrowings due to the swaps (as a percent)					0.35%	0.37%