Convertible preferred stock (Details)	1 Months Ended					12 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	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2018 EUR (€) € / shares shares	Dec. 31, 2018 USD ($) shares
Convertible preferred stock
Aggregate number of authorized shares for all classes of equity						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						0			612,446	612,446
Series A convertible preferred stock | Shares issued to related parties
Convertible preferred stock
Number of convertible preferred shares issued during the period					145,469
Proceeds from issuance of convertible preferred shares | $					$ 3,233,417
Number of convertible preferred shares issued during the period as settlement of debt				466,977
Value of convertible preferred shares issued as settlement of debt | $				$ 3,249,627
Convertible preferred stock, issued						612,446
Series B convertible preferred stock
Convertible preferred stock
Number of convertible preferred shares issued during the period								3,322,762
Proceeds from issuance of convertible preferred shares | $								$ 23,343,863
Convertible preferred stock, issued						0			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 | $										$ 8,806,426
Convertible preferred stock, liquidation preference | $										$ 58,874,347
Percentage of outstanding shares									50.00%	50.00%
Series B convertible preferred stock | Shares issued to related parties
Convertible preferred stock
Number of convertible preferred shares issued during the period			3,322,762	3,322,762
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				590,445
Value of convertible preferred shares issued as settlement of debt | $				$ 4,108,818
Convertible preferred stock, issued						7,235,969
Series C convertible preferred stock
Convertible preferred stock
Number of convertible preferred shares issued during the period						467,179	4,712,698
Proceeds from issuance of convertible preferred shares | $						$ 5,598,362	$ 56,364,645
Convertible preferred stock, issued						0			4,712,698	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 | $										$ 389,970
Convertible preferred stock, liquidation preference | $										$ 57,115,312
Percentage of outstanding shares									50.00%	50.00%
Series C convertible preferred stock | Shares issued to related parties
Convertible preferred stock
Number of convertible preferred shares issued during the period	467,173	4,712,698
Proceeds from issuance of convertible preferred shares | $	$ 5,651,872	$ 56,725,342
Convertible preferred stock, issued						4,219,854