Debt - Other Outstanding Letters of Credit (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Line Of Credit Facility [Line Items]
Other outstanding letters of credit	$ 246	$ 188
Letters of Credit Outstanding - Recorded in Balance Sheet
Line Of Credit Facility [Line Items]
Other outstanding letters of credit	$ 215	$ 161
Committed and Uncommitted Lines
Line Of Credit Facility [Line Items]
Maturity year	2018