Employee Postretirement Benefits - Schedule of Amounts Recognized in Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit costs and other current liabilities	$ (7)	$ (1)
Deferred credits and other liabilities	(30)	(80)
Net amount recognized at end of year	(37)	(81)
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit costs and other current liabilities	(10)	(9)
Deferred credits and other liabilities	(154)	(139)
Net amount recognized at end of year	$ (164)	$ (148)