Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7	$ 8	$ 8
Interest cost	117	110	119
Expected return on plan assets	(159)	(179)	(173)
Amortization of unrecognized net loss	45	42	43
Settlements and curtailments	1	9	11
Net periodic benefit cost	11	(10)	8
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1	1
Interest cost	2	2	2
Expected return on plan assets	(3)	(5)	(5)
Amortization of unrecognized net loss	(1)	(1)
Amortization of unrecognized prior service benefit		(2)	(2)
Net periodic benefit cost	$ (2)	$ (5)	$ (4)