Summary of Significant Accounting Policies - Allowance for Doubtful Accounts Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance of allowance for doubtful accounts	$ 8	$ 7	$ 6	$ 7
Bad debt expense	10	8	4
Charge-offs	(9)	(7)	(5)
Ending balance of allowance for doubtful accounts	$ 8	$ 7	$ 6