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, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	$ 1		$ 1	$ 3	$ 3
Interest cost on projected benefit obligation	14		12	38	37
Expected return on assets	(16)		(16)	(49)	(45)
Amortization of net actuarial loss	7		6	21	19
Pension expense	$ 6		$ 3	$ 13	$ 14
Contributions by Employer		$ 46