Fair Value Measurements and Disclosures - Assets and Liabilities Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Financial Assets
Mutual Fund Investments	$ 70	$ 71
Commodity Derivative Instruments	12	0
Financial Liabilities Fair Value Disclosure [Abstract]
Commodity Derivative Instruments	(6)	(116)
Portion of Deferred Compensation Liability Measured at Fair Value	(89)	(88)
Stock Based Compensation Liability Measured at Fair Value	(11)	(9)
Quoted Prices in Active Markets (Level 1)
Financial Assets
Mutual Fund Investments	70	71
Commodity Derivative Instruments	0	0
Financial Liabilities Fair Value Disclosure [Abstract]
Commodity Derivative Instruments	0	0
Portion of Deferred Compensation Liability Measured at Fair Value	(89)	(88)
Stock Based Compensation Liability Measured at Fair Value	(11)	(9)
Significant Other Observable Inputs (Level 2)
Financial Assets
Mutual Fund Investments	0	0
Commodity Derivative Instruments	16	5
Financial Liabilities Fair Value Disclosure [Abstract]
Commodity Derivative Instruments	(10)	(121)
Portion of Deferred Compensation Liability Measured at Fair Value	0	0
Stock Based Compensation Liability Measured at Fair Value	0	0
Significant Unobservable Inputs (Level 3)
Financial Assets
Mutual Fund Investments	0	0
Commodity Derivative Instruments	0
Financial Liabilities Fair Value Disclosure [Abstract]
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
Scenario, Adjustment
Financial Assets
Mutual Fund Investments	0	0
Commodity Derivative Instruments	(4)	(5)
Financial Liabilities Fair Value Disclosure [Abstract]
Commodity Derivative Instruments	4	5
Portion of Deferred Compensation Liability Measured at Fair Value	0	0
Stock Based Compensation Liability Measured at Fair Value	$ 0	$ 0