Organization, Basis of Presentation and Summary of Significant Accounting Policies - Impact of Adoption of ASC on Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
ASSETS
Property and equipment, net	$ 18,281		$ 78,723
Operating lease right-of-use assets	28,190
Prepaid rent	33,559
Liabilities:
Operating lease liabilities - current	1,320
Deferred rent			271
Build-to-suit lease obligation	13,299		27,689
Accumulated deficit	$ (603,949)		(562,696)
Build-to-Suit Lease [Member]
Liabilities:
Build-to-suit lease obligation			$ 27,689
Topic 842 [Member]
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
Build-to-suit lease obligation		7,933
Accumulated deficit		(561,799)
Topic 842 [Member] | Adjustments Due to the Adoption of ASC Topic 842 [Member]
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)
Build-to-suit lease obligation		7,933
Accumulated deficit		897
Topic 842 [Member] | Build-to-Suit Lease [Member] | Adjustments Due to the Adoption of ASC Topic 842 [Member]
Liabilities:
Build-to-suit lease obligation		$ (27,689)