Employee Benefit Plans and Equity Compensation Plan (Changes in Benefit Obligations) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, January 1	$ 732	$ 793
Service cost	15	17	$ 18
Interest cost	28	29	32
Plan participants’ contributions	0	0
Actuarial (gain) loss	47	(46)
Benefits paid	(21)	(19)
Settlements	(80)	(42)
Curtailment	6	0
Amounts funded to parent	0	0
Benefit obligation, December 31	727	732	793
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, January 1	187	217
Service cost	3	4	4
Interest cost	9	8	9
Plan participants’ contributions	1	1
Actuarial (gain) loss	22	(31)
Benefits paid	(13)	(11)
Settlements	0	0
Curtailment	3	0
Amounts funded to parent	2	(1)
Benefit obligation, December 31	$ 214	$ 187	$ 217