Risk Management - Fair Value of Derivative Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Asset derivatives	$ 460	$ 551
Liability derivatives	(65)	(171)
Energy commodity derivative contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	56	221
Liability derivatives	(58)	(50)
Interest rate contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	358	133
Liability derivatives	(1)	(115)
Foreign currency contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	46	197
Liability derivatives	(6)	(6)
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Asset derivatives	452	529
Liability derivatives	(58)	(166)
Designated as Hedging Instrument | Energy commodity derivative contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	48	199
Liability derivatives	(51)	(45)
Designated as Hedging Instrument | Energy commodity derivative contracts | Fair Value of Derivatives Contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	31	135
Designated as Hedging Instrument | Energy commodity derivative contracts | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Liability derivatives	(43)	(45)
Designated as Hedging Instrument | Energy commodity derivative contracts | Deferred Charges and Other Assets
Derivatives, Fair Value [Line Items]
Asset derivatives	17	64
Designated as Hedging Instrument | Energy commodity derivative contracts | Other Long-Term Liabilities and Deferred Credits
Derivatives, Fair Value [Line Items]
Liability derivatives	(8)	0
Designated as Hedging Instrument | Interest rate contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	358	133
Liability derivatives	(1)	(115)
Designated as Hedging Instrument | Interest rate contracts | Fair Value of Derivatives Contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	45	12
Designated as Hedging Instrument | Interest rate contracts | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Liability derivatives	0	(37)
Designated as Hedging Instrument | Interest rate contracts | Deferred Charges and Other Assets
Derivatives, Fair Value [Line Items]
Asset derivatives	313	121
Designated as Hedging Instrument | Interest rate contracts | Other Long-Term Liabilities and Deferred Credits
Derivatives, Fair Value [Line Items]
Liability derivatives	(1)	(78)
Designated as Hedging Instrument | Foreign currency contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	46	197
Liability derivatives	(6)	(6)
Designated as Hedging Instrument | Foreign currency contracts | Fair Value of Derivatives Contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	0	91
Designated as Hedging Instrument | Foreign currency contracts | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Liability derivatives	(6)	(6)
Designated as Hedging Instrument | Foreign currency contracts | Deferred Charges and Other Assets
Derivatives, Fair Value [Line Items]
Asset derivatives	46	106
Designated as Hedging Instrument | Foreign currency contracts | Other Long-Term Liabilities and Deferred Credits
Derivatives, Fair Value [Line Items]
Liability derivatives	0	0
Not Designated as Hedging Instrument | Energy commodity derivative contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	8	22
Liability derivatives	(7)	(5)
Not Designated as Hedging Instrument | Energy commodity derivative contracts | Fair Value of Derivatives Contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	8	22
Not Designated as Hedging Instrument | Energy commodity derivative contracts | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Liability derivatives	$ (7)	$ (5)