Fair Value Measurements Proved and Unproved Oil and Gas Properties (Details) - USD ($)		3 Months Ended				9 Months Ended		12 Months Ended
		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net		$ 5,417,083,000				$ 5,417,083,000		$ 6,081,354,000
Impairment of Proved Oil and Gas Properties		0			$ 8,049,000	3,806,000	$ 277,834,000
Abandonment and impairment of unproved properties		$ 0			$ 3,568,000	157,000	$ 5,917,000
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						$ 526,500,000
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down			$ 166,900,000	$ 359,600,000
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%
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]							$ 88,205,000

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