Fair Value Measurements Fair Value Measurements (Nonrecurring Fair Value Measurements) (Details) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2015 USD ($) $ / MMBTU $ / bbl	Mar. 31, 2015 USD ($) $ / MMBTU $ / bbl	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Inventory Write-down	$ 12,000			$ 20,000
Impairment of oil and gas properties	$ (72,000)		$ 0	(210,000)	$ 0
Management oil price outlook | $ / bbl	57.41	65.02
Management gas price outlook | $ / MMBTU	3.46	3.83
West Panhandle [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Asset Group		$ 61,000
Impairment of oil and gas properties		$ (138,000)
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)