Fair Value Measurements (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Financial Assets:
Financial Assets: Natural Gas Options/Swaptions	$ 53	$ 100
Financial Assets: Crude Oil Swaps	54	121
Financial Assets: Crude Oil Options/Swaptions		244
Financial Liabilities:
Proved Oil and Gas Properties, Other Property, Plant and Equipment and Other Assets Carrying Amount	7
Proved Oil and Gas Properties, Other Property, Plant and Equipment and Other Assets Written Down During the Period - Fair value at End of Period	2
Pretax Impairment Charges for Proved Oil and Gas Properties, Other Property, Plant and Equipment and Other Assets	5
Aggregate Principal Amount of Current and Long-Term Debt	6,390	5,890
Estimated Fair Value of Debt	6,655	6,242
Fair Value, Inputs, Level 1 [Member]
Financial Assets:
Financial Assets: Natural Gas Options/Swaptions	0	0
Financial Assets: Crude Oil Swaps	0	0
Financial Assets: Crude Oil Options/Swaptions		0
Fair Value, Inputs, Level 2 [Member]
Financial Assets:
Financial Assets: Natural Gas Options/Swaptions	53	100
Financial Assets: Crude Oil Swaps	54	121
Financial Assets: Crude Oil Options/Swaptions		244
Fair Value, Inputs, Level 3 [Member]
Financial Assets:
Financial Assets: Natural Gas Options/Swaptions	0	0
Financial Assets: Crude Oil Swaps	$ 0	0
Financial Assets: Crude Oil Options/Swaptions		$ 0