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, 2016		Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016		Jun. 30, 2015	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, Plant and Equipment, Net	$ 4,601,332					$ 4,601,332			$ 4,950,280
Period of New York Mercantile Exchange Strip Pricing Used for Price Forecast						5 years
Impairment of Proved Oil and Gas Properties	0			$ 12,914		$ 269,785		$ 68,440	468,700
Abandonment and impairment of unproved properties	38			$ 5,819		2,349		$ 17,446	78,600
Impairment of Long-Lived Assets Held-for-use									49,400
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down	(49,500)		$ 68,300		$ 30,000
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	$ 99,900	[1]				$ 99,900	[1]		$ 124,800	[2]
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%

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

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