Derivatives (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013 Derivatives	Dec. 31, 2012
Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 25,000
Weighted-average Fixed Rate Swapped	1.70%
Interest rate swap [Member]
Derivatives, Fair Value [Line Items]
Fair Value	(37,687)	(42,537)
Notional Amount	1,437,000
Number of Swaps	40
Weighted-average Fixed Rate Swapped	1.54%
Interest rate swap [Member] | 0-12 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	300,000
Number of Swaps	5
Weighted-average Fixed Rate Swapped	1.29%
Interest rate swap [Member] | 13-36 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	490,000
Number of Swaps	11
Weighted-average Fixed Rate Swapped	1.76%
Interest rate swap [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	247,000
Number of Swaps	10
Weighted-average Fixed Rate Swapped	1.29%
Interest rate swap [Member] | Over 60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	400,000
Number of Swaps	14
Weighted-average Fixed Rate Swapped	1.60%
Interest rate swap [Member] | Hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value	(35,314)	(39,813)
Notional Amount	1,410,000	1,435,000
Weighted-average Fixed Rate Swapped	1.51%	1.50%
Interest rate swap [Member] | Hedging instrument [Member] | 0-12 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	300,000
Interest rate swap [Member] | Hedging instrument [Member] | 13-36 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	490,000
Interest rate swap [Member] | Hedging instrument [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	220,000
Interest rate swap [Member] | Hedging instrument [Member] | Over 60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	400,000
Interest rate swap [Member] | Designated as Trading [Member]
Derivatives, Fair Value [Line Items]
Fair Value	(2,373)	(2,724)
Notional Amount	27,000	27,000
Weighted-average Fixed Rate Swapped	2.88%	2.88%
Interest rate swap [Member] | Designated as Trading [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 27,000