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Variable Interest Entities (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Variable Interest Entities (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Assets		$ 26,619	$ 27,743
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Assets		110	109
Refined Coal Facility [Member]
Variable Interest Entity [Line Items]
Equity method investments impairment charges	$ 7	7
Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Aggregate investment balance		99	101
Variable Interest Entity, Not Primary Beneficiary [Member] \| Investments Qualifying for Federal Tax Credits [Member]
Variable Interest Entity [Line Items]
Aggregate investment balance		260	309
Variable Interest Entity, Not Primary Beneficiary [Member] \| Investment In Low Income Housing Properties [Member]
Variable Interest Entity [Line Items]
Equity method investments debt balance		$ 240	$ 269