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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2	$ 2	$ 5	$ 5
Interest cost	4	4	7	7
Expected return on plan assets	(5)	(4)	(10)	(9)
Amortization of actuarial loss and prior service cost	0	1	1	2
Net periodic benefit cost	$ 1	$ 3	$ 3	$ 5