Employee Benefit Plans (Fair Values of Pension and Post Retirement Plan Assets by Asset Category) (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Common stock	$ 365	$ 508
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Pending sales of securities and advanced subscriptions	198
Pending sales of securities		52
Net accrued income	20	24
Pending purchases of securities	71	99
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Pending sales of securities	10	2
Net accrued income	2	2
Pending purchases of securities	$ 2	$ 5