Derivatives and Other Financial Instruments - Schedule of Reconciliation of Activity for Derivative Contracts (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021
Power Contract [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Beginning balance	$ 1,397	$ 814
Other comprehensive (loss) income (unrealized)	212	902
Fair value measurement, Liabilities, Ending balance	1,530	1,530
Power Contract [Member] | Sales [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities	(79)	(186)
Financial Contracts [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Beginning balance		1
Fair value measurement, Liabilities, Cost of goods sold		(8)
Other comprehensive (loss) income (unrealized)		6
Fair value measurement, Liabilities, Other	1	3
Fair value measurement, Liabilities, Ending balance	8	8
Fair value measurement, Assets, Beginning balance	5
Other comprehensive (loss) income (unrealized)	(2)
Fair value measurement, assets, Other	6	6
Financial Contracts [Member] | Cost of Goods Sold [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(6)	(6)
Financial Contracts [Member] | Other Expense (Income), Net [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities	7	6
Fair value measurement, Assets	(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	8	7
Fair value measurement, Assets, Other income, net	(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, Ending balance	17	17
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)	(6)
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)	$ (5)