Derivatives (Details) (Interest rate swap [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2012 Derivatives	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative Liability, Number of Instruments Held	7
Fair Value	$ (39,267)	$ (27,997)
Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	1,512,000
Number of Swaps	42
Weighted-average Fixed Rate Swapped	1.53%
Derivative liabilities [Member] | 0-12 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	75,000
Number of Swaps	2
Weighted-average Fixed Rate Swapped	1.30%
Derivative liabilities [Member] | 13-36 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	565,000
Number of Swaps	10
Weighted-average Fixed Rate Swapped	1.44%
Derivative liabilities [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	287,000
Number of Swaps	11
Weighted-average Fixed Rate Swapped	2.04%
Derivative liabilities [Member] | Over 60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	585,000
Number of Swaps	19
Weighted-average Fixed Rate Swapped	1.39%
Hedging instrument [Member] | Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair Value	(36,551)	(25,512)
Cumulative Notional Amount	1,485,000	1,065,000
Notional Amount	1,485,000
Weighted-average Fixed Rate Swapped	1.50%	1.55%
Hedging instrument [Member] | Derivative liabilities [Member] | 0-12 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	75,000
Hedging instrument [Member] | Derivative liabilities [Member] | 13-36 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	565,000
Hedging instrument [Member] | Derivative liabilities [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	260,000
Hedging instrument [Member] | Derivative liabilities [Member] | Over 60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	585,000
Trading instrument [Member] | Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair Value	(2,716)	(2,485)
Cumulative Notional Amount		27,000
Notional Amount	27,000
Weighted-average Fixed Rate Swapped	2.88%	2.88%
Trading instrument [Member] | Derivative liabilities [Member] | 0-12 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	0
Trading instrument [Member] | Derivative liabilities [Member] | 13-36 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	0
Trading instrument [Member] | Derivative liabilities [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Cumulative Notional Amount	27,000
Trading instrument [Member] | Derivative liabilities [Member] | Over 60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	0
Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Cumulative Notional Amount	275,000
Weighted-average Fixed Rate Swapped	1.62%
Forward Contracts, March 25, 2013 Start Date [Member]
Derivatives, Fair Value [Line Items]
Cumulative Notional Amount	100,000
Weighted-average Fixed Rate Swapped	1.89%
Forward Contracts, April 15, 2013 Start Date [Member]
Derivatives, Fair Value [Line Items]
Cumulative Notional Amount	25,000
Weighted-average Fixed Rate Swapped	1.70%
Forward Contracts, January 8, 2013 Start Date [Member]
Derivatives, Fair Value [Line Items]
Cumulative Notional Amount	$ 150,000
Weighted-average Fixed Rate Swapped	0.77%