Employee Benefit Plans and Equity Compensation Plan (Schedule of Defined Benefit Plan Amounts Recognized in Regulatory Assets) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets, pension and other postretirement benefit plans, net unamortized gain (loss) arising during the period, net of tax	$ 1	$ (51)	$ 41
Regulatory assets, amortization of actuarial losses, pension and other postretirement benefit plans, net of tax	(6)	(11)	(10)
Regulatory assets, settlement loss, pension and other postretirement benefit plans, net of tax	(6)	(16)	0
Regulatory assets, total recognized in regulatory assets, pension and other postretirement benefit plans, net of tax	(11)	(78)	31
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets, pension and other postretirement benefit plans, net unamortized gain (loss) arising during the period, net of tax	(27)	20	(26)
Regulatory assets, amortization of actuarial losses, pension and other postretirement benefit plans, net of tax	(1)		(1)
Regulatory assets, settlement loss, pension and other postretirement benefit plans, net of tax	0	0	0
Regulatory assets, total recognized in regulatory assets, pension and other postretirement benefit plans, net of tax	$ (28)	$ 20	$ (27)