Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 2	$ 3	$ 4
Interest cost	30	28	59	55
Expected return on plan assets	(40)	(45)	(80)	(90)
Amortization of unrecognized net loss	12	10	23	21
Settlement charge	2	3	2	7
Net periodic (benefit) cost	5	(2)	7	(3)
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost			1	1
Expected return on plan assets	(1)	(1)	(2)	(2)
Amortization of unrecognized prior service benefit				(1)
Net periodic (benefit) cost	$ (1)	$ (1)	$ (1)	$ (2)