Derivative Financial Instruments Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018		Dec. 31, 2017
Fair value of derivative assets and liabilities
Derivative, Fair Value, Net	$ (122,400)		$ (139,400)
Derivative Assets, Current	77,296		64,266
Derivative Assets, Noncurrent	35,128		40,362
Total Derivative Assets	112,424		104,628
Derivative Liabilities, Current	181,068		172,582
Derivatives Liabilities, Noncurrent	53,712		71,402
Total Derivative Liabilities	234,780		243,984
Derivative Asset, Not Offset in the Accompanying Balance Sheets	(100,617)		(100,035)
Derivative Liabilities, Not Offset in the Accompanying Balance Sheets	100,617		100,035
Derivative Asset, Fair Value, Net Amounts	11,807		4,593
Derivative Liabilities, Fair Value, Net Amounts	(134,163)		(143,949)
Not Designated as Hedging Instrument [Member]
Fair value of derivative assets and liabilities
Derivative Assets, Current	77,296		64,266
Derivative Assets, Noncurrent	35,128		40,362
Derivative Liabilities, Current	181,068		172,582
Derivatives Liabilities, Noncurrent	53,712		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	112,424	[1]	104,628	[2]
Total Derivative Liabilities	$ 234,780	[1]	$ 243,984	[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.