Commitments and contingencies - Development and supply agreement and Legal contingencies (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares shares
Commitments and contingencies
Amount payable to counterparty	$ 11.7
Period to purchase minimum number of commercial units	7 years
Fee payable on shortfall of commercial units purchase	$ 9.6
Issuance of supply warrants on termination of agreement, number (in shares) | shares	93,341
Issuance of supply warrants on termination of agreement, value per share (in dollars per share) | $ / shares	$ 0.003214