Convertible preferred stock (Details)	1 Months Ended					6 Months Ended	9 Months Ended
	Feb. 28, 2019 USD ($) shares	Nov. 30, 2018 USD ($) shares	Oct. 31, 2017 USD ($) $ / shares shares	Mar. 31, 2016 USD ($) shares	Jun. 30, 2011 USD ($) shares	Jun. 30, 2019 USD ($) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2019 EUR (€) € / shares shares	Sep. 30, 2019 USD ($) shares	Dec. 31, 2018 shares
Convertible preferred stock
Aggregate number of authorized shares for all classes of equity | shares								440,000,000	440,000,000
Criteria for automatic conversion of Series A preferred shares
Convertible preferred stock
Threshold multiplier for determining automatic conversion								1.5	1.5
Average price paid per Series C convertible preferred share | € / shares								€ 16.05
Minimum aggregate proceeds | €								€ 50,000,000
Criteria for automatic conversion of Series B preferred shares
Convertible preferred stock
Threshold multiplier for determining automatic conversion								1.5	1.5
Average price paid per Series C convertible preferred share | € / shares								€ 16.05
Minimum aggregate proceeds | €								€ 50,000,000
Criteria for automatic conversion of Series C preferred shares
Convertible preferred stock
Threshold multiplier for determining automatic conversion								1.5	1.5
Average price paid per Series C convertible preferred share | € / shares								€ 16.05
Minimum aggregate proceeds | €								€ 50,000,000
Series A convertible preferred stock
Convertible preferred stock
Convertible preferred stock, issued | shares								612,446	612,446	612,446
Convertible preferred stock, liquidation preference | $									$ 6,483,044
Series A convertible preferred stock | Shares issued to related parties
Convertible preferred stock
Number of convertible preferred shares issued during the period | shares					145,472
Proceeds from issuance of convertible preferred shares | $					$ 3,233,417
Number of convertible preferred shares issued during the period as settlement of debt | shares				466,974
Value of convertible preferred shares issued as settlement of debt | $				$ 3,249,627
Convertible preferred stock, issued | shares								612,446	612,446
Series B convertible preferred stock
Convertible preferred stock
Convertible preferred stock, issued | shares								7,235,969	7,235,969	7,235,969
Dividends to which each holder is entitled (as a percent)							8.00%
Dividends declared (paid) during the period | $							$ 0
Accrued dividends added to liquidation preference amount | $									$ 12,220,935
Convertible preferred stock, liquidation preference | $									$ 62,288,856
Series B convertible preferred stock | Shares issued to related parties
Convertible preferred stock
Number of convertible preferred shares issued during the period | shares			3,322,759	3,322,764
Temporary equity, price per share | $ / shares			$ 7.02
Proceeds from issuance of convertible preferred shares | $			$ 23,349,617	$ 22,609,485
Number of convertible preferred shares issued during the period as settlement of debt | shares				590,446
Value of convertible preferred shares issued as settlement of debt | $				$ 4,108,818
Convertible preferred stock, issued | shares								7,235,969	7,235,969
Series C convertible preferred stock
Convertible preferred stock
Number of convertible preferred shares issued during the period | shares						467,179
Proceeds from issuance of convertible preferred shares | $						$ 5,598,362
Convertible preferred stock, issued | shares								5,179,877	5,179,877	4,712,698
Dividends to which each holder is entitled (as a percent)							8.00%
Dividends declared (paid) during the period | $							$ 0
Accrued dividends added to liquidation preference amount | $									$ 4,074,601
Convertible preferred stock, liquidation preference | $									$ 66,451,815
Series C convertible preferred stock | Shares issued to related parties
Convertible preferred stock
Number of convertible preferred shares issued during the period | shares	467,173	4,712,704
Proceeds from issuance of convertible preferred shares | $	$ 5,651,872	$ 56,725,342
Convertible preferred stock, issued | shares								4,219,854	4,219,854