Derivatives and Risk Mangement (Balance Sheet Classification) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Total derivative assets	$ 571,962	$ 717,343
Total derivative liabilities	7,255	12,513
Designated as Hedging Instrument [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	567,546	713,640
Derivative Liability Designated as Hedging Instrument, Fair Value	7,091	12,058
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Hedging Asset [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	338,499	333,479
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Other Assets
Derivative Asset Designated as Hedging Instrument, Fair Value	127,072	201,081
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	7,086	11,849
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Asset [Member]
Derivative Asset Designated as Hedging Instrument, Fair Value	101,975	179,080
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Liability [Member]
Derivative Liability Designated as Hedging Instrument, Fair Value	5	209
Not Designated as Hedging Instrument [Member]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	4,416	3,703
Derivative Liability Not Designated as Hedging Instrument, Fair Value	164	455
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Asset [Member]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	2,987	1,906
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Assets
Derivative Asset Not Designated as Hedging Instrument, Fair Value	1,429	1,797
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Liability [Member]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	104	400
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Long-Term Hedging Liability [Member]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	$ 60	$ 55