Employee Benefit Plans and Equity Compensation Plan (Changes in Benefit Obligations) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, January 1	$ 727	$ 732
Service cost	12	15	$ 17
Interest cost	24	28	29
Plan participants’ contributions	0	0
Actuarial (gain) loss	41	47
Benefits paid	(22)	(21)
Settlements	(40)	(80)
Curtailment	0	6
Amounts funded to parent	0	0
Benefit obligation, December 31	742	727	732
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, January 1	214	187
Service cost	3	3	4
Interest cost	8	9	8
Plan participants’ contributions	2	1
Actuarial (gain) loss	(31)	22
Benefits paid	(13)	(13)
Settlements	0	0
Curtailment	0	3
Amounts funded to parent	1	2
Benefit obligation, December 31	$ 184	$ 214	$ 187