Fair Value Measurements and Disclosures - Assets and Liabilities Measured on a Nonrecurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Financial Assets
Mutual Fund Investments	$ 38	$ 57
Commodity Derivative Instruments	180	2
Financial Liabilities
Commodity Derivative Instruments	(27)	(73)
Portion of Deferred Compensation Liability Measured at Fair Value	(43)	(71)
Stock Based Compensation Liability Measured at Fair Value	(8)	(10)
Quoted Prices in Active Markets (Level 1)
Financial Assets
Mutual Fund Investments	38	57
Commodity Derivative Instruments	0	0
Financial Liabilities
Commodity Derivative Instruments	0	0
Portion of Deferred Compensation Liability Measured at Fair Value	(43)	(71)
Stock Based Compensation Liability Measured at Fair Value	(8)	(10)
Significant Other Observable Inputs (Level 2)
Financial Assets
Mutual Fund Investments	0	0
Commodity Derivative Instruments	187	7
Financial Liabilities
Commodity Derivative Instruments	(34)	(78)
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	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
Adjustment
Financial Assets
Mutual Fund Investments	0	0
Commodity Derivative Instruments	(7)	(5)
Financial Liabilities
Commodity Derivative Instruments	7	5
Portion of Deferred Compensation Liability Measured at Fair Value	0	0
Stock Based Compensation Liability Measured at Fair Value	$ 0	$ 0