Commitments and contingencies - Development and supply agreement and Legal contingencies (Details) - STRATEC Biomedical - Development and supply agreement $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 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	$ 10.3
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