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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 12	$ 12	$ 24	$ 24
Expected return on assets	(14)	(14)	(28)	(28)
Amortization of net actuarial loss	5	5	11	12
Administrative costs	1	1	2	2
Net periodic postretirement benefit cost	$ 4	$ 4	$ 9	$ 10