Debt - Other Outstanding Letters of Credit (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Line Of Credit Facility [Line Items]
Other outstanding letters of credit	$ 284	$ 246
Committed Lines
Line Of Credit Facility [Line Items]
Other outstanding letters of credit	29	29
Uncommitted Lines
Line Of Credit Facility [Line Items]
Other outstanding letters of credit	$ 255	$ 217