Significant Accounting Policies - Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) Performance_Obligation	Dec. 31, 2019 USD ($)
Significant Accounting Policies [Line Items]
Restricted cash	$ 2,138	$ 2,138
Operating lease right-of-use assets	37,733
Operating lease liabilities	46,335
Accumulated deficit	$ (467,042)	$ (365,478)
ASU 2016-02 (Topic 842)
Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, early adoption	false
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, immaterial effect	false
Operating lease right-of-use assets	$ 33,000
Operating lease liabilities	39,700
Derecognition of deferred rent	7,300
Reclassification of tenant incentive receivables	500
Derecognized portion of build-to-suit asset	36,600
Accumulated deficit	$ 100
Accounting Standards Update 2018-15
Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, early adoption	false
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, immaterial effect	true
Accounting Standards Update 2019-12
Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
Change in accounting principle, accounting standards update, early adoption	true
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, immaterial effect	true
Sequencing Revenue
Significant Accounting Policies [Line Items]
Number of revenue performance obligations | Performance_Obligation	2