Pension and Other Postretirement Benefits - Components of Pension Expense for Defined Benefit Pension Plan (Detail) - Pension Plan, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Interest cost on projected benefit obligation	$ 48	$ 49	$ 51
Expected return on assets	(58)	(63)	(62)
Amortization of net actuarial loss	28	27	24
Administrative costs	3	4	5
Net periodic postretirement benefit cost	21	$ 17	$ 18
Out of period adjustment	$ 5