Commitments and contingencies (Narratives) (Details)	1 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($) kg	Dec. 31, 2020 USD ($)
Commitment And Contingencies [Line Items]
Short-term purchase commitment, minimum mass required | kg	400
Losses for contingent liability		$ 0
Provision for a loss contingency		0
PCAs
Commitment And Contingencies [Line Items]
Purchase obligation	$ 5,900,000	$ 0