Fair Value Measurements and Disclosures - Assets and Liabilities Measured on a Nonrecurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	$ 70	$ 92	$ 1,285
Total Pre-tax (Non-cash) Impairment Loss, Impaired oil and gas properties	$ 70	92	533
Impaired Materials and Supplies Inventory		105	81
Total Pre-tax (Non-cash) Impairment Loss, Materials and supplies inventory		14	20
Discount rate for impairment model	10.00%
Quoted Prices in Active Markets (Level 1)
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)
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
Commodity Derivative Instruments		0
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	0	0	752
Impaired Materials and Supplies Inventory		91	$ 61
Fair Value, Measurements, Recurring
Financial Assets
Mutual Fund Investments	57	71
Commodity Derivative Instruments	2	0
Financial Liabilities
Commodity Derivative Instruments	(73)	(116)
Portion of Deferred Compensation Liability Measured at Fair Value	(71)	(88)
Stock Based Compensation Liability Measured at Fair Value	(10)	(9)
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Financial Assets
Mutual Fund Investments	57	71
Commodity Derivative Instruments	0	0
Financial Liabilities
Commodity Derivative Instruments	0	0
Portion of Deferred Compensation Liability Measured at Fair Value	(71)	(88)
Stock Based Compensation Liability Measured at Fair Value	(10)	(9)
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Financial Assets
Mutual Fund Investments	0	0
Commodity Derivative Instruments	7	5
Financial Liabilities
Commodity Derivative Instruments	(78)	(121)
Portion of Deferred Compensation Liability Measured at Fair Value	0	0
Stock Based Compensation Liability Measured at Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Financial Assets
Mutual Fund Investments	0	0
Commodity Derivative Instruments	0
Financial Liabilities
Commodity Derivative Instruments	0	0
Portion of Deferred Compensation Liability Measured at Fair Value	0	0
Stock Based Compensation Liability Measured at Fair Value	0	0
Fair Value, Measurements, Recurring | Adjustment
Financial Assets
Mutual Fund Investments	0	0
Commodity Derivative Instruments	(5)	(5)
Financial Liabilities
Commodity Derivative Instruments	5	5
Portion of Deferred Compensation Liability Measured at Fair Value	0	0
Stock Based Compensation Liability Measured at Fair Value	$ 0	$ 0