Derivatives and Risk Mangement (Balance Sheet Classification) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Total derivative assets	$ 292,144	$ 717,343
Total derivative liabilities	4,266	12,513
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	287,993	713,640
Derivative Liability Designated as Hedging Instrument, Fair Value		12,058
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	279,443	333,479
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	8,550	201,081
Designated as Hedging Instrument [Member] | Fixed and Floating Price Swaps [Member] | Hedging Liability [Member]
Derivative [Line Items]
Derivative Liability Designated as Hedging Instrument, Fair Value		11,849
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value		179,080
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Liability [Member]
Derivative [Line Items]
Derivative Liability Designated as Hedging Instrument, Fair Value		209
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	4,151	3,703
Derivative Liability Not Designated as Hedging Instrument, Fair Value	4,266	455
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	3,250	1,906
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative Asset Not Designated as Hedging Instrument, Fair Value	901	1,797
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Liability [Member]
Derivative [Line Items]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	138	400
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Long-Term Hedging Liability [Member]
Derivative [Line Items]
Derivative Liability Not Designated as Hedging Instrument, Fair Value		55
Not Designated as Hedging Instrument [Member] | Call Options Written [Member] | Long-Term Hedging Liability [Member]
Derivative [Line Items]
Derivative Liability Not Designated as Hedging Instrument, Fair Value	$ 4,128