Summary of significant accounting policies - Net income (loss) per share - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	6,064,344	5,949,201	4,460,446
Numerator:
Net income (loss)	$ 36,741	$ (43,597)	$ (88,288)
Denominator for basic net income (loss) per share:
Weighted average shares outstanding-basic	37,132,447	35,898,266	32,065,781
Denominator for diluted net income (loss) per share:
Weighted average shares outstanding-basic	37,132,447	35,898,266	32,065,781
Common stock options and restricted stock units	216,068
Weighted average shares outstanding-diluted	37,348,514	35,898,266	32,065,781
Financial instruments with off-balance sheet risk	$ 0
Unvested restricted common stock awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	156,863	176,471	235,294
Unvested restricted common stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	527,625	455,404
Outstanding stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities	5,379,856	5,317,326	4,225,152