Summary of Significant Accounting Policies - Schedule of Impact of Adoption of ASC 842 (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating lease right-of-use assets, net	$ 6,385
Property and equipment, net	379		$ 7,290
Construction financing liability			5,342
Operating lease liabilities, current	406
Operating lease liabilities, net of current portion	4,822
Accumulated deficit	$ (183,151)		(168,493)
Prior to ASC 842 Adoption [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Property and equipment, net			7,290
Construction financing liability			5,342
Other liabilities, current			3,639
Accumulated deficit			$ (168,493)
ASU No. 2016-02 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating lease right-of-use assets, net		$ 6,697
Property and equipment, net		292
Other liabilities, current		3,552
Operating lease liabilities, current		334
Operating lease liabilities, net of current portion		5,067
Accumulated deficit		(168,766)
ASU No. 2016-02 [Member] | ASC 842 Adjustment [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating lease right-of-use assets, net		6,697
Property and equipment, net		(6,998)
Construction financing liability		(5,342)
Other liabilities, current		(87)
Operating lease liabilities, current		334
Operating lease liabilities, net of current portion		5,067
Accumulated deficit		$ (273)