Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Financial Assets:
Financial Assets: Crude Oil Swaps	$ 11	$ 65
Financial Assets: Crude Oil Options/Swaptions	4	36
Financial Assets: Natural Gas Options/Swaptions	18	65
Financial Liabilities:
Financial Liabilities: Crude Oil Options/Swaptions	45	8
Financial Liabilities: Natural Gas Options/Swaptions	14	13
Financial Liabilities: Foreign Currency Rate Swap	49	55
Financial Liabilities: Interest Rate Swap	3	4
Proved Oil and Gas Property and Other Assets Carrying Amount	68
Proved and Unproved Oil and Gas Properties and Other Property, Plant and Equipment Written Down During the Period - Fair value at End of Period	41
Pretax Impairment Charge for Proved Oil and Gas Properties and Other Property, Plant and Equipment	27
Aggregate Principal Amount of Current and Long-Term Debt	6,290	6,290
Fair Value of Debt	6,980	7,032
Fair Value, Inputs, Level 1 [Member]
Financial Assets:
Financial Assets: Crude Oil Swaps	0	0
Financial Assets: Crude Oil Options/Swaptions	0	0
Financial Assets: Natural Gas Options/Swaptions	0	0
Financial Liabilities:
Financial Liabilities: Crude Oil Options/Swaptions	0	0
Financial Liabilities: Natural Gas Options/Swaptions	0	0
Financial Liabilities: Foreign Currency Rate Swap	0	0
Financial Liabilities: Interest Rate Swap	0	0
Fair Value, Inputs, Level 2 [Member]
Financial Assets:
Financial Assets: Crude Oil Swaps	11	65
Financial Assets: Crude Oil Options/Swaptions	4	36
Financial Assets: Natural Gas Options/Swaptions	18	65
Financial Liabilities:
Financial Liabilities: Crude Oil Options/Swaptions	45	8
Financial Liabilities: Natural Gas Options/Swaptions	14	13
Financial Liabilities: Foreign Currency Rate Swap	49	55
Financial Liabilities: Interest Rate Swap	3	4
Fair Value, Inputs, Level 3 [Member]
Financial Assets:
Financial Assets: Crude Oil Swaps	0	0
Financial Assets: Crude Oil Options/Swaptions	0	0
Financial Assets: Natural Gas Options/Swaptions	0	0
Financial Liabilities:
Financial Liabilities: Crude Oil Options/Swaptions	0	0
Financial Liabilities: Natural Gas Options/Swaptions	0	0
Financial Liabilities: Foreign Currency Rate Swap	0	0
Financial Liabilities: Interest Rate Swap	$ 0	$ 0