Stockholders' equity - Summary of Expense Related to Revalued Options (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation		$ 872,177	$ 263,396	$ 122,134
Accelerated share-based compensation summary [Member] | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options	153,940
Share-based compensation	$ 134,258
Accelerated share-based compensation summary [Member] | Grant Date 1/11/2011 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options	16,667
Accelerated share-based compensation summary [Member] | Grant Date 1/20/2014 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options	6,667
Accelerated share-based compensation summary [Member] | Grant Date 2/27/2018 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options	60,606
Accelerated share-based compensation summary [Member] | Grant Date 4/21/2017 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options	20,000
Accelerated share-based compensation summary [Member] | Grant Date 4/30/2019 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options	50,000
Previously recognized expense | Accelerated share-based compensation summary [Member] | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 414,018
Previously recognized expense | Accelerated share-based compensation summary [Member] | Grant Date 1/11/2011 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	109,769
Previously recognized expense | Accelerated share-based compensation summary [Member] | Grant Date 1/20/2014 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	120,712
Previously recognized expense | Accelerated share-based compensation summary [Member] | Grant Date 2/27/2018 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	99,738
Previously recognized expense | Accelerated share-based compensation summary [Member] | Grant Date 4/21/2017 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	70,346
Previously recognized expense | Accelerated share-based compensation summary [Member] | Grant Date 4/30/2019 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	13,453
Accelerated vesting expense | Accelerated share-based compensation summary [Member] | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	87,783
Accelerated vesting expense | Accelerated share-based compensation summary [Member] | Grant Date 1/11/2011 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	0
Accelerated vesting expense | Accelerated share-based compensation summary [Member] | Grant Date 1/20/2014 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	0
Accelerated vesting expense | Accelerated share-based compensation summary [Member] | Grant Date 2/27/2018 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	0
Accelerated vesting expense | Accelerated share-based compensation summary [Member] | Grant Date 4/21/2017 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	20,736
Accelerated vesting expense | Accelerated share-based compensation summary [Member] | Grant Date 4/30/2019 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	67,047
Incremental vesting expense | Accelerated share-based compensation summary [Member] | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	46,475
Incremental vesting expense | Accelerated share-based compensation summary [Member] | Grant Date 1/11/2011 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	11,782
Incremental vesting expense | Accelerated share-based compensation summary [Member] | Grant Date 1/20/2014 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	7,621
Incremental vesting expense | Accelerated share-based compensation summary [Member] | Grant Date 2/27/2018 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	12,313
Incremental vesting expense | Accelerated share-based compensation summary [Member] | Grant Date 4/21/2017 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	13,485
Incremental vesting expense | Accelerated share-based compensation summary [Member] | Grant Date 4/30/2019 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	1,274
Total of previous, accelerated and incremental expense | Accelerated share-based compensation summary [Member] | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	548,276
Total of previous, accelerated and incremental expense | Accelerated share-based compensation summary [Member] | Grant Date 1/11/2011 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	121,551
Total of previous, accelerated and incremental expense | Accelerated share-based compensation summary [Member] | Grant Date 1/20/2014 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	128,333
Total of previous, accelerated and incremental expense | Accelerated share-based compensation summary [Member] | Grant Date 2/27/2018 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	112,051
Total of previous, accelerated and incremental expense | Accelerated share-based compensation summary [Member] | Grant Date 4/21/2017 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	104,567
Total of previous, accelerated and incremental expense | Accelerated share-based compensation summary [Member] | Grant Date 4/30/2019 | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 81,774