Deferred Revenue - Additional Information (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Deferred revenue current liabilities	$ 34	$ 35	$ 34
Deferred revenue non-current liabilities	111	$ 104	99
ETC contracts
Disaggregation of Revenue [Line Items]
Deferred revenue current liabilities	27		29
Deferred revenue non-current liabilities	$ 93		$ 77