Summary of Significant Accounting Policies - Allowance for Doubtful Accounts Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance of allowance for doubtful accounts	$ 6	$ 7	$ 2	$ 1
Bad debt expense	4	5	2
Charge-offs	(5)	0	(1)
Ending balance of allowance for doubtful accounts	$ 6	$ 7	$ 2