Debt - Other Outstanding Letters of Credit (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Line of Credit Facility [Line Items]
Other outstanding letters of credit	$ 269	$ 272
Committed Lines
Line of Credit Facility [Line Items]
Other outstanding letters of credit	54	54
Uncommitted Lines
Line of Credit Facility [Line Items]
Other outstanding letters of credit	$ 215	$ 218