Financing - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended						1 Months Ended	12 Months Ended
	Aug. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Maximum	Aug. 31, 2011 Line of Credit	Dec. 31, 2010 Master Notes	Nov. 30, 2009 Master Notes Interest Rate Lock Commitments	Dec. 31, 2010 Master Notes Senior Unsecured Notes, 3.81 Percent, Due May 2018	Dec. 31, 2010 Master Notes Senior Unsecured Notes, 4.60 Percent, Due May 2021
Line of Credit Facility [Line Items]
Loan agreement expiration date	2016-08	2018-01
Revolving loan facility, maximum capacity	$ 350,000,000					$ 250,000,000
Term Loan outstanding amount				200,000,000
Repayment of term loan	100,000,000
Issuance of senior notes									25,000,000	75,000,000
Interest rate for purchase agreement									3.81%	4.60%
Maturity date									May 2018	May 2021
Issuance date of senior notes							2011-05
Gain on interest rate lock settlement								251,000
Deferred tax related to interest rate lock settlement		(28,901,000)	(16,946,000)	(16,888,000)				149,000
Repayment of acquired borrowings		4,647,000
Capital lease obligations		7,179,000	7,253,000
Capital lease obligations, current		2,887,000	2,653,000
Letter of credit outstanding		29,358,000
Debt outstanding from dealers		$ 981,584,000
Aggregate repurchase obligation, amount, percentage					15.00%