Debt - Outstanding Letters of Credit (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Line Of Credit Facility [Line Items]
Contingent liabilities under letters of credit outstanding	$ 32	$ 54
Letters of credit outstanding, balance sheet liabilities	$ 81	$ 343
Committed and Uncommitted Lines
Line Of Credit Facility [Line Items]
Maturity year	2016