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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2	$ 2	$ 5	$ 5
Interest cost	3	4	6	7
Expected return on plan assets	(6)	(5)	(11)	(10)
Amortization of actuarial loss and prior service cost	1	0	1	1
Net periodic benefit cost	$ 0	$ 1	$ 1	$ 3