Summary of Significant Accounting Policies - Impact of Adoption of ASC 842 (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment, net	$ 4,245	$ 4,976	$ 52,835
Operating lease right-of-use assets	24,607	28,530	0
Deferred tax assets	9,459	8,708	9,702
Lease liability, current	5,002	4,717	0
Other current liabilities	20,078	5,882	5,926
Build-to-suit lease obligation, current	0	0	1,645
Lease liability, non-current	19,161	22,939	0
Build-to-suit lease obligation, non-current	0	0	49,901
Deferred rent	0	0	176
Accumulated deficit	(650,331)	(594,208)	$ (597,995)
Total Operating Lease liability	$ 24,162
ASC-842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment, net		(47,859)
Operating lease right-of-use assets		28,530
Cumulative effect of new accounting pronouncement		(994)
Lease liability, current		4,717
Other current liabilities		(44)
Build-to-suit lease obligation, current		(1,645)
Lease liability, non-current		22,939
Build-to-suit lease obligation, non-current		(49,901)
Deferred rent		(176)
Accumulated deficit		3,787
Discount rate, percent	4.25%
Total Operating Lease liability		27,700
Built-to-suit leases, asset		47,900
Built-to-suit leases, liability		$ 51,500