Derivatives (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012 Derivatives	Dec. 31, 2011
Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 275,000
Weighted-average Fixed Rate Swapped	1.62%
Derivative Liability, Number of Instruments Held	7
Interest rate swap [Member]
Derivatives, Fair Value [Line Items]
Fair Value	(42,537)	(27,997)
Notional Amount	1,462,000
Weighted-average Fixed Rate Swapped	1.53%
Number of Swaps	41
Interest rate swap [Member] | 0-12 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	25,000
Weighted-average Fixed Rate Swapped	0.84%
Number of Swaps	1
Interest rate swap [Member] | 13-36 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	715,000
Weighted-average Fixed Rate Swapped	1.52%
Number of Swaps	15
Interest rate swap [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	322,000
Weighted-average Fixed Rate Swapped	1.51%
Number of Swaps	11
Interest rate swap [Member] | Over 60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	400,000
Weighted-average Fixed Rate Swapped	1.60%
Number of Swaps	14
Interest rate swap [Member] | Hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Fair Value	(39,813)	(25,512)
Notional Amount	1,435,000	1,065,000
Weighted-average Fixed Rate Swapped	1.50%	1.55%
Interest rate swap [Member] | Hedging instrument [Member] | 0-12 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	25,000
Interest rate swap [Member] | Hedging instrument [Member] | 13-36 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	715,000
Interest rate swap [Member] | Hedging instrument [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	295,000
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	(2,724)	(2,485)
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