Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands		3 Months Ended								12 Months Ended
	Sep. 20, 2018	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Reverse stock split	0.1449
Accumulated deficit		$ 126,402				$ 61,627				$ 126,402	$ 61,627
Net loss		$ 17,338	$ 18,588	$ 15,473	$ 13,376	17,368	$ 9,451	$ 7,790	$ 6,768	64,775	41,377	$ 18,750
Net cash used in operating activities										38,162	34,971	15,292
Impairment losses										$ 0	0	$ 0
Lessor financed improvement incentives due but not yet received						$ 1,200					$ 1,200
Percentage of realized upon ultimate settlement										50.00%
Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Lease, right-of-use asset													$ 14,200
Lease liability													11,200
Derecognition of deferred rent													1,800
Derecognition of building and related accumulated depreciation													14,500
Derecognition of financing lease obligation													$ 10,500