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, 2016	Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	$ 1	$ 1
Interest cost on projected benefit obligation	12	12
Expected return on assets	(14)	(16)
Amortization of net actuarial loss	7	7
Net periodic postretirement benefit cost	$ 6	$ 4