Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of short-term and long-term debt and lease obligation.

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Variable Interest Entities (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Variable Interest Entities (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Carrying value of debt		$ 13,810	$ 13,498
Assets		29,345	27,743	$ 22,650
Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Aggregate investment balance		228	309
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Assets		114	109
Refined Coal Facility [Member]
Variable Interest Entity [Line Items]
Equity method investments impairment charges	$ 7	7
Refined Coal Facility [Member] \| Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Equity method investments impairment charges	$ 7
Investment In Low Income Housing Properties [Member] \| Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Equity method investments debt balance		210	269
Trust For Final Capping, Closure, Post-closure Or Environmental Remediation Obligations [Member] \| Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Aggregate investment balance		$ 106	$ 101