Commitments and contingencies - Long-term Commitments (Details) $ in Millions	Dec. 31, 2020 USD ($)
Management Service Commitments
Long-term Purchase Commitments
2021	$ 0.4
2022	0.2
Total	0.6
Fuel Supply and Transportation Commitments
Long-term Purchase Commitments
2021	4.2
2022	4.4
Total	$ 8.6

Amount of purchase arrangement to be paid in next fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

No definition available.

Name:	us-gaap_PurchaseObligationDueInNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant