Significant Accounting Policies - Components of Lease Expense (Paranthetical) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2019		Jun. 30, 2018	Jun. 30, 2019		Jun. 30, 2018
Schedule Of Accounting Policies [Line Items]
Operating lease expense	$ 6	[1]	$ 5	$ 12	[1]	$ 9
Sublease income	1	[1]	1	2	[1]	1
Depreciation expense	23		21	46		41
Interest expense	$ 2		3	$ 3		6
Corporate HQ Building
Schedule Of Accounting Policies [Line Items]
Depreciation expense			1			2
Interest expense			$ 2			$ 4

[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 six months ended June 30, 2018, we recorded operating lease expense of $5 million and $9 million, respectively; and sublease income of $1 million for both the three and six months ended June 30, 2018 in our unaudited condensed consolidated statement of operations in accordance with ASC 840.