Debt - Outstanding Letters of Credit (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Line Of Credit Facility [Line Items]
Contingent liabilities under letters of credit outstanding	$ 27	$ 32
Letters of credit outstanding, balance sheet liabilities	$ 161	$ 81
Committed and Uncommitted Lines
Line Of Credit Facility [Line Items]
Maturity year	2018