Fair Value Measurements Fair Value Measurements (Nonrecurring Fair Value Measurements) (Details) $ in Thousands		3 Months Ended					9 Months Ended
		Sep. 30, 2016 USD ($)	Mar. 31, 2016 USD ($) $ / MMBTU $ / bbl	Dec. 31, 2015 USD ($) $ / MMBTU $ / bbl	Sep. 30, 2015 USD ($) $ / MMBTU $ / bbl	Mar. 31, 2015 USD ($) $ / MMBTU $ / bbl	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Tangible Asset Impairment Charges	[1]	$ 1,000			$ 13,000		$ 6,000	$ 21,000
Impairment of oil and gas properties		$ 0			$ (72,000)		$ (32,000)	(210,000)
Management oil price outlook | $ / bbl			49.77	52.82	57.41	65.02
Management gas price outlook | $ / MMBTU			3.24	3.34	3.46	3.83
Impairment of Long-Lived Assets Held-for-use			$ 32,000
West Panhandle [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Asset Group			33,000			$ 61,000
Impairment of oil and gas properties			$ (32,000)			$ (138,000)
South Texas Eagle Ford Shale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Asset Group				$ 483,000
Impairment of oil and gas properties				$ (846,000)
South Texas Edwards And Austin Chalk [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Asset Group					$ 88,000			$ 88,000
Impairment of oil and gas properties					$ (72,000)

[1] Primarily represents charges to terminate rig contracts that were not required to meet planned drilling activities.