Risk Management - Fair Value of Derivative Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Asset derivatives	$ 931	$ 460
Liability derivatives	(78)	(65)
Energy commodity derivative contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	99	56
Liability derivatives	(63)	(58)
Interest rate contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	694	358
Liability derivatives	(10)	(1)
Foreign currency contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	138	46
Liability derivatives	(6)	(6)
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Asset derivatives	907	452
Liability derivatives	(57)	(58)
Designated as Hedging Instrument | Energy commodity derivative contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	75	48
Liability derivatives	(41)	(51)
Designated as Hedging Instrument | Energy commodity derivative contracts | Fair Value of Derivatives Contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	42	31
Designated as Hedging Instrument | Energy commodity derivative contracts | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Liability derivatives	(33)	(43)
Designated as Hedging Instrument | Energy commodity derivative contracts | Deferred Charges and Other Assets
Derivatives, Fair Value [Line Items]
Asset derivatives	33	17
Designated as Hedging Instrument | Energy commodity derivative contracts | Other Long-Term Liabilities and Deferred Credits
Derivatives, Fair Value [Line Items]
Liability derivatives	(8)	(8)
Designated as Hedging Instrument | Interest rate contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	694	358
Liability derivatives	(10)	(1)
Designated as Hedging Instrument | Interest rate contracts | Fair Value of Derivatives Contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	119	45
Designated as Hedging Instrument | Interest rate contracts | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Liability derivatives	(3)	0
Designated as Hedging Instrument | Interest rate contracts | Deferred Charges and Other Assets
Derivatives, Fair Value [Line Items]
Asset derivatives	575	313
Designated as Hedging Instrument | Interest rate contracts | Other Long-Term Liabilities and Deferred Credits
Derivatives, Fair Value [Line Items]
Liability derivatives	(7)	(1)
Designated as Hedging Instrument | Foreign currency contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	138	46
Liability derivatives	(6)	(6)
Designated as Hedging Instrument | Foreign currency contracts | Fair Value of Derivatives Contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	0	0
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	138	46
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 | Fair Value of Derivatives Contracts
Derivatives, Fair Value [Line Items]
Asset derivatives	24	8
Not Designated as Hedging Instrument | Energy commodity derivative contracts | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Liability derivatives	$ (21)	$ (7)