Fair Value Measurements Fair Value Measurements (Fair Value of Assets Classified as Held For Sale (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and gas properties	$ (72,000)	$ 0	$ (210,000)	$ 0
Hugoton field [Member] | Discontinued Operations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Asset Group		328,000		328,000
Impairment of oil and gas properties		(34,000)		(34,000)
Barnett Shale Field [Member] | Discontinued Operations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Asset Group		149,000		149,000
Impairment of oil and gas properties		(46,000)		(174,000)
Pioneer Alaska [Member] | Discontinued Operations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Asset Group					$ 253,000
Impairment of oil and gas properties		$ 0		$ (97,000)