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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3	$ 3	$ 9	$ 8
Interest cost	3	4	10	11
Expected return on plan assets	(4)	(5)	(13)	(13)
Amortization of actuarial loss and prior service cost	1	1	4	2
Net periodic benefit cost	$ 3	$ 3	$ 10	$ 8