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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3	$ 3	$ 8	$ 8
Interest cost	4	4	11	11
Expected return on plan assets	(6)	(5)	(16)	(14)
Amortization of actuarial loss and prior service cost	1	0	2	2
Net periodic benefit cost	$ 2	$ 2	$ 5	$ 7