Net loss per share - Antidilutive Securities (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potentially dilutive securities (in shares)	16,610,721	9,018,214	16,610,721	9,018,214
Stock options outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potentially dilutive securities (in shares)	5,707,634	7,727,158	5,707,634	7,727,158
Unvested restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potentially dilutive securities (in shares)	3,427,190	1,291,056	3,427,190	1,291,056
Shares related to the 2026 Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potentially dilutive securities (in shares)	7,475,897	0	7,475,897	0