Interest Rate Swaps (Details) (Interest Rate Swap Agreements, USD $) In Thousands, unless otherwise specified	Dec. 31, 2012 Fair Value, Inputs, Level 2	Dec. 31, 2011 Fair Value, Inputs, Level 2	Dec. 31, 2012 Floating Rate Floor Plan Debt	Dec. 31, 2012 Floating Rate Floor Plan Debt LIBOR portion	Jan. 31, 2011 Floating Rate Floor Plan Debt LIBOR portion	Dec. 31, 2010 Floating Rate Floor Plan Debt LIBOR portion	Dec. 31, 2012 Floating Rate Floor Plan Debt Fixed rate portion
Derivative
Portion of floating rate floor plan debt fixed by swap agreements				$ 300,000	$ 300,000	$ 300,000	$ 100,000
Interest rate swap, fixed (as a percent)				2.135%	3.67%	3.67%	1.55%
Estimated liabilities of swaps designated as hedging instruments, fair value	$ 14,337	$ 15,952
Increase in the weighted average interest rate on floor plan borrowings due to the swaps (as a percent)			0.38%