Employee Benefit Plans (AOCI and Regulatory Assets and Liabilities that have Not been Recognized as Components of Periodic Benefit (Credit) Cost) (Detail) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss		$ 1,717	$ 1,793
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss		3,207	3,327
Prior service (credit) cost		4	5
Total	[1]	3,211	3,332
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss		120	241
Prior service (credit) cost		(232)	(339)
Total	[1]	$ (112)	$ (98)

[1] As of December 31, 2020, of the $3.2 billion and $(112) million related to pension benefits and other postretirement benefits, $1.9 billion and $(40) million, respectively, are included in AOCI, with the remainder included in regulatory assets and liabilities. As of December 31, 2019, of the $3.3 billion and $(98) million related to pension benefits and other postretirement benefits, $2.0 billion and $(65) million, respectively, are included in AOCI, with the remainder included in regulatory assets and liabilities.