Derivative Financial Instruments Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2015		Dec. 31, 2014
Fair value of derivative assets and liabilities
Derivative, Fair Value, Net	$ 488,403		$ 592,138
Derivative Assets, Current	367,710		402,668
Derivative Liabilities, Current	8		0
Derivative Assets, Noncurrent	120,701		189,540
Derivative Liabilities, Noncurrent	0		70
Derivative Asset, Fair Value, Gross Asset	488,411		592,208
Derivative Liability, Fair Value, Gross Liability	8		70
Derivative Asset, Not Offset, Policy Election Deduction	(8)		(70)
Derivative Liability, Not Offset, Policy Election Deduction	8		70
Derivative Asset, Fair Value, Amount Offset Against Collateral, Net	488,403		592,138
Derivative Liability, Fair Value, Amount Offset Against Collateral, Net	0		0
Not Designated as Hedging Instrument [Member]
Fair value of derivative assets and liabilities
Derivative Assets, Current			402,668
Derivative Liabilities, Current			0
Derivative Assets, Noncurrent			189,540
Derivative Liabilities, Noncurrent			70
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value of derivative assets and liabilities
Derivative Asset, Fair Value, Gross Asset	488,411	[1]	592,208	[2]
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Not Designated as Hedging Instrument [Member]
Fair value of derivative assets and liabilities
Derivative Asset, Fair Value, Gross Asset	488,411		592,208
Derivative Liability, Fair Value, Gross Liability	$ 8	[1]	$ 70	[2]

[1] This represents a financial asset or liability that is measured at fair value on a recurring basis.

[2] This represents a financial asset or liability that is measured at fair value on a recurring basis.