Derivatives (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012 Derivatives	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Weighted-average Fixed Rate Swapped	1.62%
Notional Amount	$ 275,000
Interest rate swap [Member]
Derivatives, Fair Value [Line Items]
Weighted-average Fixed Rate Swapped	1.53%
Fair Value	(46,496)	(27,997)
Derivative Liability, Number of Instruments Held	7
Notional Amount	1,512,000
Number of Swaps	42
Interest rate swap [Member] | 0-12 months [Member]
Derivatives, Fair Value [Line Items]
Weighted-average Fixed Rate Swapped	1.30%
Notional Amount	75,000
Number of Swaps	2
Interest rate swap [Member] | 13-36 months [Member]
Derivatives, Fair Value [Line Items]
Weighted-average Fixed Rate Swapped	1.44%
Notional Amount	565,000
Number of Swaps	10
Interest rate swap [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Weighted-average Fixed Rate Swapped	1.60%
Notional Amount	472,000
Number of Swaps	16
Interest rate swap [Member] | Over 60 months [Member]
Derivatives, Fair Value [Line Items]
Weighted-average Fixed Rate Swapped	1.60%
Notional Amount	400,000
Number of Swaps	14
Interest rate swap [Member] | Hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Cumulative Notional Amount	1,485,000	1,065,000
Weighted-average Fixed Rate Swapped	1.50%	1.55%
Fair Value	(43,802)	(25,512)
Notional Amount	1,485,000
Interest rate swap [Member] | Hedging instrument [Member] | 0-12 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	75,000
Interest rate swap [Member] | Hedging instrument [Member] | 13-36 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	565,000
Interest rate swap [Member] | Hedging instrument [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	445,000
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]
Cumulative Notional Amount		27,000
Weighted-average Fixed Rate Swapped	2.88%	2.88%
Fair Value	(2,694)	(2,485)
Notional Amount	27,000
Interest rate swap [Member] | Designated as Trading [Member] | 0-12 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	0
Interest rate swap [Member] | Designated as Trading [Member] | 13-36 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	0
Interest rate swap [Member] | Designated as Trading [Member] | 37-60 months [Member]
Derivatives, Fair Value [Line Items]
Cumulative Notional Amount	27,000
Interest rate swap [Member] | Designated as Trading [Member] | Over 60 months [Member]
Derivatives, Fair Value [Line Items]
Notional Amount	$ 0