Pension and Other Postretirement Benefits - Components of Pension Expense for Defined Benefit Pension Plan (Detail) - Pension Plan, Defined Benefit [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 12	$ 14	$ 36	$ 38
Expected return on assets	(15)	(16)	(43)	(49)
Amortization of net actuarial loss	9	7	21	21
Administrative costs	1	1	3	3
Net periodic postretirement benefit cost	7	$ 6	17	$ 13
Out of period adjustment	$ 5		$ 5