Interest Rate Swaps (Details) (Interest Rate Swap Agreements, USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Derivative
Hedge ineffectiveness recorded	$ 0	$ 0
Fair Value, Inputs, Level 2
Derivative
Estimated liabilities of swaps designated as hedging instruments, fair value	12,598,000	14,337,000
Floating Rate Floor Plan Debt
Derivative
Increase in the weighted average interest rate on floor plan borrowings due to the swaps (as a percent)	0.38%
Floating Rate Floor Plan Debt | LIBOR portion
Derivative
Portion of floating rate floor plan debt fixed by swap agreements	300,000,000
Interest rate swap, fixed (as a percent)	2.135%
Floating Rate Floor Plan Debt | Fixed rate portion
Derivative
Portion of floating rate floor plan debt fixed by swap agreements	$ 100,000,000
Interest rate swap, fixed (as a percent)	1.55%