Employee Postretirement Benefits - Schedule of Amounts Recognized in Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit costs and other current liabilities	$ (1)	$ (1)
Deferred credits and other liabilities	(15)	(42)
Net amount recognized at end of year	(16)	(43)
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit costs and other current liabilities	(12)	(11)
Deferred credits and other liabilities	(186)	(156)
Net amount recognized at end of year	$ (198)	$ (167)