Significant Accounting Policies - Additional Information (Details) $ in Millions		1 Months Ended	3 Months Ended	9 Months Ended
		Jun. 30, 2013 ft²	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule Of Accounting Policies [Line Items]
Operating lease right-of-use assets			$ 76	$ 76
Operating lease liabilities			85	85
Leased area | ft²		280,000
Finance lease liability			84	84
ROU asset, net of accumulated amortization	[1]		2	7
Accumulated amortization			$ 296	$ 296		$ 240
Corporate HQ Building
Schedule Of Accounting Policies [Line Items]
Lease expiration date		Dec. 01, 2030
Initial term of lease		15 years 7 months
Extended lease term		5 years
Accounting Standards Update 2016-02
Schedule Of Accounting Policies [Line Items]
Operating lease right-of-use assets	[2]				$ 75
Operating lease liabilities					88
Operating lease, term			6 years	6 years
Operating lease, existence of option to extend				true
Operating lease, existence of option to terminate				true
Operating lease, Option to terminate				within 1 year
Finance lease ROU assets					114
Finance lease liability					88
Finance lease prepaid rent					26
Accounting Standards Update 2016-02 | Property and Equipment, Net
Schedule Of Accounting Policies [Line Items]
ROU asset, net of accumulated amortization				$ 107
Accounting Standards Update 2016-02 | Corporate Headquarters Lease
Schedule Of Accounting Policies [Line Items]
Net asset					26
Accumulated amortization			$ 7	$ 7
Accounting Standards Update 2016-02 | Maximum
Schedule Of Accounting Policies [Line Items]
Lease expiration date				Jun. 30, 2027
Accounting Standards Update 2016-02 | Difference between Leases Guidance in Effect before and after Topic 842
Schedule Of Accounting Policies [Line Items]
Operating lease right-of-use assets					88
Deferred rent					$ (13)

[1] 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.

[2] Refer to the below discussion regarding the transition accounting for operating and finance leases upon adoption of ASC 842.