Significant Accounting Policies - Components of Lease Expense (Paranthetical) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019		Sep. 30, 2018	Sep. 30, 2019		Sep. 30, 2018
Schedule Of Accounting Policies [Line Items]
Operating lease expense	$ 6	[1]	$ 5	$ 18	[1]	$ 15
Sublease income	1	[1]	1	3	[1]	2
Depreciation expense	23		20	69		61
Interest expense	$ 2		4	$ 5		10
Corporate HQ Building
Schedule Of Accounting Policies [Line Items]
Depreciation expense			1			2
Interest expense			$ 2			$ 5

[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.