Employee Postretirement Benefits - Schedule of Amounts Recognized in Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit costs and other current liabilities	$ (5)	$ (7)
Deferred credits and other liabilities	(68)	(30)
Net amount recognized at end of year	(73)	(37)
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit costs and other current liabilities	(10)	(10)
Deferred credits and other liabilities	(137)	(154)
Net amount recognized at end of year	$ (147)	$ (164)