Organization, Basis of Presentation and Summary of Significant Accounting Policies - Impact of Adoption of ASC on Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
ASSETS
Property and equipment, net	$ 26,516		$ 78,723
Operating lease right-of-use assets	77,505
Liabilities:
Operating lease liabilities - current	2,994
Deferred rent			271
Operating lease liabilities - long-term	40,609		27,689
Accumulated deficit	$ (661,540)		(562,696)
Topic 842
ASSETS
Property and equipment, net		$ 16,223
Operating lease right-of-use assets		8,753
Prepaid rent		36,025
Liabilities:
Operating lease liabilities - current		1,408
Operating lease liabilities - long-term		7,933
Accumulated deficit		(561,799)
Topic 842 | Adjustments Due to the Adoption of ASC Topic 842
ASSETS
Property and equipment, net		(62,500)
Operating lease right-of-use assets		8,753
Prepaid rent		36,025
Liabilities:
Operating lease liabilities - current		1,408
Deferred rent		(271)
Operating lease liabilities - long-term		7,933
Accumulated deficit		897
Build To Suit Lease
Liabilities:
Operating lease liabilities - long-term			$ 27,689
Build To Suit Lease | Topic 842 | Adjustments Due to the Adoption of ASC Topic 842
Liabilities:
Operating lease liabilities - long-term		$ (27,689)