Commitments and contingencies - Leases (Details)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2017 ft² item	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2019 USD ($)
Future minimum lease payments
Rent expenses		$ 100,000	$ 100,000	$ 200,000	$ 200,000
Capital leases
Interest expense				7,000	$ 34,000
Office and laboratory space
Operating leases
Lease term	62 months
Space of leased area | ft²	11,000
Number of additional extension of lease terms | item	2
Additional lease term	5 years
Lessee, Operating Lease, Existence of Option to Terminate [true false]	false
Deferred rent		11,000		11,000				$ 18,000
Future minimum lease payments
2020 (represents three remaining months)		56,000		56,000
2021		225,000		225,000
2022		153,000		153,000
Total		$ 434,000		$ 434,000
Lab equipment
Capital leases
Lease Monthly payments						$ 32,000	$ 32,000
Lab equipment | Minimum
Capital leases
Finance lease term						36 months	36 months
Lease Monthly payments						$ 2,000	$ 2,000
Interest rates						9.43%	9.43%
Lab equipment | Maximum
Capital leases
Finance lease term						38 months	38 months
Interest rates						11.35%	11.35%