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
	Mar. 31, 2017	Mar. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 12	$ 12
Expected return on assets	(14)	(14)
Amortization of net actuarial loss	6	7
Administrative costs	1	1
Net periodic postretirement benefit cost	$ 5	$ 6