Debt (Details) (USD $)						1 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 [InterestRateSwapMember]	Mar. 31, 2013 [AssetBackedSecuritizationTermNotesMember]	Dec. 31, 2012 [AssetBackedSecuritizationTermNotesMember]	Feb. 28, 2013 [AssetBackedSecuritizationTermNotesMember] [ClassAFixedRateSecuredNotesMember]	Feb. 28, 2013 [AssetBackedSecuritizationTermNotesMember] [ClassBFixedRateSecuredNotesMember]	Mar. 31, 2013 [SecuredDebtMember]	Dec. 31, 2012 [SecuredDebtMember]	Dec. 31, 2012 [AssetBackedWarehouseFacilityMember]	Mar. 31, 2013 [LineOfCreditMember]	Dec. 31, 2012 [LineOfCreditMember]	Mar. 31, 2013 [CapitalLeaseObligationsMember]	Dec. 31, 2012 [CapitalLeaseObligationsMember]
Debt
Total debt	$ 2,711,602,000	$ 2,604,015,000		$ 1,559,357,000	$ 1,343,826,000			$ 762,767,000	$ 698,570,000	$ 355,000,000	$ 266,000,000	$ 75,000,000	$ 123,478,000	$ 131,619,000
Debt outstanding on facilities with fixed interest rates	1,514,800,000
Debt outstanding on facilities with interest rates based on floating rate indices	1,196,800,000
Net notional amount			765,800,000
Notes issued						253,000,000	18,800,000
Borrowing capacity											$ 450,000,000