Revenue from contracts - Disaggregation of revenue (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)
Disaggregation of revenue
Total revenue	$ 72.8	$ 73.0
Number of reportable segments \| segment	4
Solid Fuel
Disaggregation of revenue
Total revenue	$ 23.4	19.8
Natural Gas
Disaggregation of revenue
Total revenue	30.8	34.6
Hydroelectric
Disaggregation of revenue
Total revenue	18.4	18.4
Corporate
Disaggregation of revenue
Total revenue	0.2	0.2
Energy sales
Disaggregation of revenue
Project revenue	40.7	37.0
Total revenue	40.7	37.0
Energy sales \| Solid Fuel
Disaggregation of revenue
Project revenue	16.8	10.4
Energy sales \| Natural Gas
Disaggregation of revenue
Project revenue	6.2	9.0
Energy sales \| Hydroelectric
Disaggregation of revenue
Project revenue	17.7	17.6
Energy capacity revenue
Disaggregation of revenue
Project revenue	28.0	30.2
Total revenue	28.0	30.2
Energy capacity revenue \| Solid Fuel
Disaggregation of revenue
Project revenue	6.3	9.3
Energy capacity revenue \| Natural Gas
Disaggregation of revenue
Project revenue	21.7	20.9
Steam energy and capacity revenue
Disaggregation of revenue
Project revenue	2.7	0.6
Steam energy and capacity revenue \| Natural Gas
Disaggregation of revenue
Project revenue	2.7	0.6
Waste heat revenue
Disaggregation of revenue
Project revenue	0.3
Waste heat revenue \| Solid Fuel
Disaggregation of revenue
Project revenue	0.3
Ancillary and transmission services
Disaggregation of revenue
Project revenue	1.4	5.1
Ancillary and transmission services \| Natural Gas
Disaggregation of revenue
Project revenue	0.7	4.3
Ancillary and transmission services \| Hydroelectric
Disaggregation of revenue
Project revenue	0.7	0.8
Asset management and operation
Disaggregation of revenue
Project revenue	0.2	0.2
Asset management and operation \| Solid Fuel
Disaggregation of revenue
Project revenue		0.1
Asset management and operation \| Natural Gas
Disaggregation of revenue
Project revenue		(0.1)
Asset management and operation \| Corporate
Disaggregation of revenue
Project revenue	0.2	0.2
Miscellaneous revenue
Disaggregation of revenue
Miscellaneous revenue	(0.5)	(0.1)
Total revenue	4.1	5.8
Miscellaneous revenue \| Natural Gas
Disaggregation of revenue
Miscellaneous revenue	$ (0.5)	$ (0.1)

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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