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	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financial Assets
Mutual Fund Investments	$ 90	$ 71	$ 90
Commodity Derivative Instruments	592	0	592
Financial Liabilities
Commodity Derivative Instruments	0	(116)	0
Stock Based Compensation Liability Measured at Fair Value	(98)	(88)	(98)
Fair Value Liabilities Measured on a Recurring Basis Stock Compensation Liability		(9)
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	5	92	533	$ 500
Oil and Gas Property, Net Book Value		92	1,285
Impaired Materials and Supplies Inventory		14	20
Material and Supply Inventory, Net Book Value		$ 105	81
Impaired Oil and Gas Properties, Net Book Value				600
Discount Rate for Impairment Model		10.00%
Quoted Prices in Active Markets (Level 1)
Financial Assets
Mutual Fund Investments	90	$ 71	90
Commodity Derivative Instruments	0	0	0
Financial Liabilities
Commodity Derivative Instruments	0	0	0
Stock Based Compensation Liability Measured at Fair Value	(98)	(88)	(98)
Fair Value Liabilities Measured on a Recurring Basis Stock Compensation Liability		(9)
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties		0	0	0
Impaired Materials and Supplies Inventory		0	0
Significant Other Observable Inputs (Level 2)
Financial Assets
Mutual Fund Investments	0	0	0
Commodity Derivative Instruments	600	5	600
Financial Liabilities
Commodity Derivative Instruments	(8)	(121)	(8)
Stock Based Compensation Liability Measured at Fair Value	0	0	0
Fair Value Liabilities Measured on a Recurring Basis Stock Compensation Liability		0
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties		0	0	0
Impaired Materials and Supplies Inventory		0	0
Significant Unobservable Inputs (Level 3)
Financial Assets
Mutual Fund Investments	0	0	0
Commodity Derivative Instruments		0
Financial Liabilities
Commodity Derivative Instruments	0	0	0
Stock Based Compensation Liability Measured at Fair Value	0	0	0
Fair Value Liabilities Measured on a Recurring Basis Stock Compensation Liability		0
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties		0	752	$ 100
Impaired Materials and Supplies Inventory		91	61
Adjustment
Financial Assets
Mutual Fund Investments	0	0	0
Commodity Derivative Instruments	(8)	(5)	(8)
Financial Liabilities
Commodity Derivative Instruments	8	5	8
Stock Based Compensation Liability Measured at Fair Value	$ 0	0	$ 0
Fair Value Liabilities Measured on a Recurring Basis Stock Compensation Liability		$ 0