Cash, Cash Equivalents and Restricted Cash - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents		$ 580	$ 390
Restricted cash and cash equivalents	[1]	325	18
Total cash, cash equivalents and restricted cash shown in the Consolidated Statements of Cash Flows		$ 905	$ 408	$ 434	$ 612

[1] Our restricted cash balances primarily served as cash collateral deposits for our letter of credit facilities. See Note 12, Debt, for further discussion.