Summary of significant accounting policies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2021 USD ($) Segment	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)
Financing Sources [Line Items]
Re-measurement gain (loss)	$ 50,000	$ 45,000
Number of reportable segments | Segment	1
Revenue from collaborations	$ 910,000
Property and equipment, net	$ 7,573,000			$ 8,007,000
Cash equivalent qualification, description	The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents.
Restricted cash	$ 556,000			560,000
Impairment of long lived assets	0	0
Other than temporary impairment losses, investments	0	0	$ 0
Royalties due	0
Expenses on milestone payments		$ 0
Dividends, common stock, cash	$ 0
Expected dividend yield	0.00%
US
Financing Sources [Line Items]
Property and equipment, net	$ 6,300,000			6,600,000
Belgium
Financing Sources [Line Items]
Property and equipment, net	1,300,000			$ 1,400,000
Product [Member]
Financing Sources [Line Items]
Revenue from collaborations	$ 0