Derivatives and Risk Mangement (Balance Sheet Classification) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Total derivative assets	$ 349,809	$ 717,343
Total derivative liabilities	2,056	12,513
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	346,594	713,640
Derivative Liability Designated as Hedging Instrument, Fair Value	1,818	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	254,358	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	48,252	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	1,818	11,849
Designated as Hedging Instrument [Member] | Costless Collar [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative Asset Designated as Hedging Instrument, Fair Value	43,984	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	3,215	3,703
Derivative Liability Not Designated as Hedging Instrument, Fair Value	238	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	2,519	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	696	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	223	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	$ 15	$ 55