Commitments and contingencies - Development and supply agreement and Legal contingencies (Details) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($) $ / shares shares
Commitments and contingencies
Amount payable to counterparty | $	$ 11.7
Period to purchase minimum number of commercial units	7 years
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