Employee Benefit Plans (AOCI and Regulatory Assets and Liabilities that have Not been Recognized as Components of Periodic Benefit (Credit) Cost) (Parenthetical) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total	[1]	$ 3,211	$ 3,332
Amount included in AOCI		1,900	2,000
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total	[1]	(112)	(98)
Amount included in AOCI		$ (40)	$ (65)

[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.