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Variable Interest Entities (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Variable Interest Entities (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Variable Interest Entity [Line Items]
Carrying value of debt			$ 13,047	$ 13,810
Assets			28,841	29,345
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Assets			117	114
Investment In Low Income Housing Properties [Member] \| Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Aggregate investment balance			193	228
Carrying value of debt			169	210
Refined Coal Facility [Member]
Variable Interest Entity [Line Items]
Equity method investments impairment charges	$ 7	$ 7
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			$ 107	$ 106