Deferred Revenue - Additional Information (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Deferred revenue current liabilities	$ 35	$ 34	$ 35
Deferred revenue non-current liabilities	103	$ 88	89
ETC contracts
Disaggregation of Revenue [Line Items]
Deferred revenue current liabilities	28		30
Deferred revenue non-current liabilities	$ 82		$ 73