Cash, Cash Equivalents and Restricted Cash - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Millions		Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents		$ 677	$ 520
Restricted cash and cash equivalents	[1]	333	325
Total cash, cash equivalents and restricted cash shown in the Statements of Cash Flows		$ 1,010	$ 845	$ 905	$ 408

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