Employee Retirement Benefits - Components of Net Periodic Costs for Qualified and Supplemental Pension Plans (Detail) - Qualified Pension Plans [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3	$ 3
Interest cost	3	3
Expected return on plan assets	(5)	(5)
Amortization of actuarial loss and prior service cost	0	1
Net periodic benefit cost	$ 1	$ 2