New Accounting Pronouncements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income (loss)	$ 14	$ 9	$ 20	$ 8
Total cash, cash equivalents and restricted cash	768	1,025	768	1,025	$ 1,089	$ 974
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income (loss)	$ 2	1	$ 4	3
Scenario, Adjustment | Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total cash, cash equivalents and restricted cash		$ 1		$ 1