Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2	$ 2	$ 4	$ 4
Interest cost	28	29	55	59
Expected return on plan assets	(45)	(43)	(90)	(86)
Amortization of unrecognized net loss	10	11	21	22
Settlement charge	3	1	7	3
Net periodic (benefit) cost	(2)		(3)	2
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)	(1)
Net periodic (benefit) cost	$ (1)	$ (1)	$ (2)	$ (2)