Derivatives and Other Financial Instruments - Schedule of Reconciliation of Activity for Derivative Contracts (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2021
Power Contract [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Beginning balance	$ 1,047	$ 814
Other comprehensive (loss) income (unrealized)	417	690
Fair value measurement, Liabilities, Ending balance	1,397	1,397
Power Contract [Member] | Sales [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities	(67)	(107)
Financial Contracts [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Beginning balance	15	1
Fair value measurement, Liabilities, Cost of goods sold	7	(8)
Other comprehensive (loss) income (unrealized)	(21)	6
Fair value measurement, Liabilities, Other		2
Other comprehensive (loss) income (unrealized)	2	2
Fair value measurement, Assets, Ending balance	5	5
Financial Contracts [Member] | Other Expense (Income), Net [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities	(1)	(1)
Fair value measurement, Assets	3	3
Financial Contracts [Member] | Other Income, Net [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Other income, net	(1)	(1)
Fair value measurement, Assets, Other income, net	3	3
Embedded Credit Derivative [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Beginning balance	18	23
Fair value measurement, Liabilities, Other	(1)
Fair value measurement, Liabilities, Ending balance	18	18
Embedded Credit Derivative [Member] | Other Expense (Income), Net [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities	1	(5)
Embedded Credit Derivative [Member] | Other Income, Net [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Other income, net	$ 1	$ (4)