Derivatives and Risk Management (Balance Sheet Classification) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative assets	$ 229,177	$ 292,144
Derivative liabilities	35,638	4,266
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets	120,990	287,993
Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative assets	115,039	279,443
Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative assets	5,951	8,550
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets	108,187	4,151
Derivative liabilities	35,638	4,266
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative assets	77,504
Not Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative assets	19,103
Not Designated as Hedging Instrument [Member] | Fixed Price Call Options [Member] | Long-Term Hedging Liability [Member]
Derivative [Line Items]
Derivative liabilities	30,980	4,128
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Asset [Member]
Derivative [Line Items]
Derivative assets	5,104	3,250
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative assets	679	901
Not Designated as Hedging Instrument [Member] | Basis Swaps [Member] | Hedging Liability [Member]
Derivative [Line Items]
Derivative liabilities	164	138
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Other Assets
Derivative [Line Items]
Derivative assets	5,797
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Hedging Liability [Member]
Derivative [Line Items]
Derivative liabilities	912
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Long-Term Hedging Liability [Member]
Derivative [Line Items]
Derivative liabilities	$ 3,582