Revenues - Contract Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Contract Assets
Balance at January 1, 2018	$ 3
Balance at September 30, 2018	7	$ 3
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Modification of Contract		29
Contract with Customer, Liability, Revenue Recognized	10	61
Contract Liabilities
Balance at January 1, 2018	12
Revenue recognized that was included in the deferred revenue balance at the beginning of the period	(10)	(61)
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Modification of Contract		29
Balance at September 30, 2018	$ 37	$ 12