Derivative Financial Instruments Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2016		Dec. 31, 2015
Fair value of derivative assets and liabilities
Derivative, Fair Value, Net	$ (91,708)		$ 488,403
Derivative Assets, Current	54,521		367,710
Derivative Liabilities, Current	115,464		8
Derivative Assets, Noncurrent	67,575		120,701
Derivative Liabilities, Noncurrent	98,340		0
Derivative Asset, Fair Value, Gross Asset	122,096		488,411
Derivative Liability, Fair Value, Gross Liability	213,804		8
Derivative Asset, Not Offset, Policy Election Deduction	(118,080)		(8)
Derivative Liability, Not Offset, Policy Election Deduction	118,080		8
Derivative Asset, Fair Value, Amount Offset Against Collateral, Net	4,016		488,403
Derivative Liability, Fair Value, Amount Offset Against Collateral, Net	(95,724)		0
Not Designated as Hedging Instrument [Member]
Fair value of derivative assets and liabilities
Derivative Assets, Current	54,521		367,710
Derivative Liabilities, Current	115,464		8
Derivative Assets, Noncurrent	67,575		120,701
Derivative Liabilities, Noncurrent	98,340		0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value of derivative assets and liabilities
Derivative Asset, Fair Value, Gross Asset	122,096	[1]	488,411	[2]
Derivative Liability, Fair Value, Gross Liability	213,804	[1]	8	[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	122,096		488,411
Derivative Liability, Fair Value, Gross Liability	$ 213,804		$ 8

[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.