Fair Value Measurements Proved and Unproved Oil and Gas Properties (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended		12 Months Ended
		Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net		$ 5,346,411				$ 5,346,411		$ 6,081,354
Impairment of Proved Oil and Gas Properties		3,806		$ 0		$ 3,806	$ 269,785
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down				$ (49,500)	$ 68,300
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net	[1]							$ 88,205
Oil and Gas Properties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Period of New York Mercantile Exchange Strip Pricing Used for Price Forecast						5 years
Oil and Gas Properties [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Discount Rate						10.00%		10.00%
Oil and Gas Properties [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Discount Rate						15.00%		15.00%
Divide County Disposal Group [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of Proved Oil and Gas Properties						$ 526,500
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down		$ 166,900	$ 359,600

[1] This represents a non-financial asset that is measured at fair value on a nonrecurring basis.