Fair Value Measurements and Disclosures of Assets and Liabilities Measured on a Nonrecurring Basis (Details\ - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financial Assets
Mutual Fund Investments	$ 90		$ 90	$ 111
Commodity Derivative Instruments	592		592	890
Financial Liabilities
Commodity Derivative Instruments	0		0	0
Portion of Deferred Compensation Liability Measured at Fair Value	(98)		(98)	(134)
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	5	$ 31	533	500	$ 86
Oil and Gas Property, Net Book Value			1,285	600
Impaired Materials and Supplies Inventory			20
Material and Supply Inventory, Net Book Value			$ 81
Impaired Oil and Gas Properties, Net Book Value						$ 199
Discount Rate for Impairment Model			10.00%
Quoted Prices in Active Markets (Level 1)
Financial Assets
Mutual Fund Investments	90		$ 90	111
Commodity Derivative Instruments	0		0	0
Financial Liabilities
Commodity Derivative Instruments	0		0	0
Portion of Deferred Compensation Liability Measured at Fair Value	(98)		(98)	(134)
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties			0	0	0
Impaired Materials and Supplies Inventory			0
Significant Other Observable Inputs (Level 2)
Financial Assets
Mutual Fund Investments	0		0	0
Commodity Derivative Instruments	600		600	890
Financial Liabilities
Commodity Derivative Instruments	(8)		(8)	0
Portion of Deferred Compensation Liability Measured at Fair Value	0		0	0
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties			0	0	0
Impaired Materials and Supplies Inventory			0
Significant Unobservable Inputs (Level 3)
Financial Assets
Mutual Fund Investments	0		0	$ 0
Commodity Derivative Instruments	0		0
Financial Liabilities
Commodity Derivative Instruments	0		0	$ 0
Portion of Deferred Compensation Liability Measured at Fair Value	0		0	0
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties			752	100	$ 113
Impaired Materials and Supplies Inventory			61
Adjustment
Financial Assets
Mutual Fund Investments	0		0	0
Commodity Derivative Instruments	(8)		(8)	0
Financial Liabilities
Commodity Derivative Instruments	8		8	0
Portion of Deferred Compensation Liability Measured at Fair Value	$ 0		$ 0	$ 0