Derivative and Hedging Activities - Fair Value of Recurring Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total derivatives	$ (118)	$ 130
Fair value, inputs, level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total derivatives	0	0
Fair value, inputs, level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total derivatives	(126)	118
Fair value, inputs, level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total derivatives	8	12
Commodity contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative asset	88	174
Derivative liability	(206)	(44)
Commodity contracts | Fair value, inputs, level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative asset	0	0
Derivative liability	0	0
Commodity contracts | Fair value, inputs, level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative asset	78	160
Derivative liability	(204)	(42)
Commodity contracts | Fair value, inputs, level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative asset	10	14
Derivative liability	(2)	(2)
Total derivatives	$ 8	$ 12	$ 87