Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2	$ 2
Interest cost	27	30
Expected return on plan assets	(45)	(43)
Amortization of unrecognized net loss	11	11
Settlement charge	4	2
Net periodic (benefit) cost	(1)	2
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1	1
Expected return on plan assets	(1)	(1)
Amortization of unrecognized prior service benefit	(1)	(1)
Net periodic (benefit) cost	$ (1)	$ (1)