Accumulated Other Comprehensive Income (Loss) (Schedule of Reclassifications out of AOCI by Component Net of Tax) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Operating revenue	$ (3,176)	$ (3,607)	$ (10,084)	$ (10,651)
Interest and related charges	407	306	978	1,136
Income tax expense (benefit)	35	(110)	200	(123)
Income (loss) including noncontrolling interests, net of tax	(601)	(112)	(1,850)	(513)
Other income	(133)	(286)	(732)	(327)
Deferred (Gains) and Losses on Derivatives-Hedging Activities | Amounts Reclassified From AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income from operations before income tax expense	14	251	47	287
Income tax expense (benefit)	(4)	(63)	(12)	(72)
Income (loss) including noncontrolling interests, net of tax	10	188	35	215
Deferred (Gains) and Losses on Derivatives-Hedging Activities | Amounts Reclassified From AOCI | Commodity contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Operating revenue		(8)		(22)
Purchased gas			1	3
Discontinued operations				(2)
Deferred (Gains) and Losses on Derivatives-Hedging Activities | Amounts Reclassified From AOCI | Interest rate contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Discontinued operations		230		236
Interest and related charges	14	23	46	66
Deferred (Gains) and Losses on Derivatives-Hedging Activities | Amounts Reclassified From AOCI | Foreign Currency Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Discontinued operations		6		6
Unrealized (gains) and losses on investment securities: | Amounts Reclassified From AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Realized (gains) losses on sale of securities	(4)	(3)	(7)	(21)
Income from operations before income tax expense	(4)	(3)	(7)	(21)
Income tax expense (benefit)	1	2	2	6
Income (loss) including noncontrolling interests, net of tax	(3)	(1)	(5)	(15)
Amortization of prior-service costs (credits) | Amounts Reclassified From AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other income	(5)	(5)	(15)	(16)
Amortization of actuarial losses | Amounts Reclassified From AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other income	30	36	100	97
Unrecognized pension and other postretirement benefit costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrecognized pension and other postretirement benefit costs, before tax	25	31	85	81
Unrecognized pension and other postretirement benefit costs, income tax expense (benefit)	(6)	(8)	(22)	(21)
Unrecognized pension and other postretirement benefit costs, net of tax	$ 19	$ 23	$ 63	$ 60