Debt - Other Outstanding Letters of Credit (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Line Of Credit Facility [Line Items]
Other outstanding letters of credit	$ 246	$ 188
Committed Lines
Line Of Credit Facility [Line Items]
Other outstanding letters of credit	29	1
Uncommitted Lines
Line Of Credit Facility [Line Items]
Other outstanding letters of credit	$ 217	$ 187