Significant Accounting Policies - Components of Lease Expense (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2019		Sep. 30, 2018	Sep. 30, 2019		Sep. 30, 2018
Lease Cost [Abstract]
Operating lease cost		$ 6	[1]	$ 5	$ 18	[1]	$ 15
Amortization of right-of-use assets	[2]	2			7
Interest on lease liabilities	[3]	1			3
Total finance lease cost		3			10
Sublease income		(1)	[1]	$ (1)	(3)	[1]	$ (2)
Total lease cost, net		$ 8			$ 25

[1] Operating lease costs, net of sublease income, are included within operating expenses in our unaudited condensed consolidated statement of operations. During the three and nine months ended September 30, 2018, we recorded operating lease expense of $5 million and $15 million, respectively; and sublease income of $1 million and $2 million, respectively, in our unaudited condensed consolidated statement of operations in accordance with ASC 840.

[2] Amount is included in depreciation expense in our unaudited condensed consolidated statement of operations. During the three and nine months ended September 30, 2018, we recorded depreciation expense of $1 million and $2 million, respectively, related to our Headquarters Lease in our unaudited condensed consolidated statement of operations in accordance with ASC 840.

[3] Amount is included in interest expense in our unaudited condensed consolidated statement of operations. During the three and nine months ended September 30, 2018, we recorded interest expense of $2 million and $5 million, respectively, related to our Headquarters Lease in our unaudited condensed consolidated statement of operations in accordance with ASC 840