Derivative Financial Instruments Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018		Dec. 31, 2017
Fair value of derivative assets and liabilities
Derivative, fair value, net	$ (149,400)		$ (139,400)
Derivative assets, current	146,329		64,266
Derivative assets, noncurrent	31,151		40,362
Total derivative assets	177,480		104,628
Derivative liabilities, current	259,338		172,582
Derivatives liabilities, noncurrent	67,583		71,402
Total derivative liabilities	326,921		243,984
Derivative asset, not offset in the accompanying balance sheets	(147,009)		(100,035)
Derivative liabilities, not offset in the accompanying balance sheets	147,009		100,035
Derivative asset, fair value, net amounts	30,471		4,593
Derivative liabilities, fair value, net amounts	(179,912)		(143,949)
Not Designated as Hedging Instrument [Member]
Fair value of derivative assets and liabilities
Derivative assets, current	146,329		64,266
Derivative assets, noncurrent	31,151		40,362
Derivative liabilities, current	259,338		172,582
Derivatives liabilities, noncurrent	67,583		71,402
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Not Designated as Hedging Instrument [Member]
Fair value of derivative assets and liabilities
Total derivative assets	177,480	[1]	104,628	[2]
Total derivative liabilities	$ 326,921	[1]	$ 243,984	[2]

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

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