Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expense related to pension and other postretirement benefits	$ 8	$ 53
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expense related to pension and other postretirement benefits	$ 2	$ 19