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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 12	$ 12	$ 36	$ 36
Expected return on plan assets	(15)	(15)	(43)	(43)
Amortization of net actuarial loss	5	9	16	21
Administrative costs		1	2	3
Net periodic postretirement benefit cost	$ 2	$ 7	$ 11	$ 17