Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2	$ 4	$ 4	$ 8
Interest cost	32	32	64	64
Expected return on plan assets	(44)	(49)	(88)	(97)
Amortization of unrecognized net loss	12	12	23	23
Settlement charge	1		2
Net periodic benefit cost	3	(1)	5	(2)
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost			1	1
Expected return on plan assets	(1)	(1)	(2)	(2)
Amortization of unrecognized prior service benefit	(1)	(3)	(2)	(5)
Amortization of unrecognized net loss		1		1
Net periodic benefit cost	$ (2)	$ (3)	$ (3)	$ (5)