Revenue from contracts - Disaggregation of revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of revenue
Project revenue	$ 272.0	$ 281.6
Solid Fuel
Disaggregation of revenue
Project revenue	94.5	80.0
Natural Gas
Disaggregation of revenue
Project revenue	118.2	131.8
Hydroelectric
Disaggregation of revenue
Project revenue	58.3	68.8
Corporate
Disaggregation of revenue
Project revenue	1.0	1.0
Energy sales
Disaggregation of revenue
Project revenue	137.9	138.0
Energy sales | Solid Fuel
Disaggregation of revenue
Project revenue	58.8	41.1
Energy sales | Natural Gas
Disaggregation of revenue
Project revenue	24.2	31.0
Energy sales | Hydroelectric
Disaggregation of revenue
Project revenue	54.9	65.9
Energy capacity revenue
Disaggregation of revenue
Project revenue	113.8	125.4
Energy capacity revenue | Solid Fuel
Disaggregation of revenue
Project revenue	34.6	38.7
Energy capacity revenue | Natural Gas
Disaggregation of revenue
Project revenue	79.2	86.7
Other
Disaggregation of revenue
Project revenue	20.3	18.2
Asset management and operation
Disaggregation of revenue
Project revenue	1.0	1.0
Asset management and operation | Corporate
Disaggregation of revenue
Project revenue	1.0	1.0
Ancillary and transmission services
Disaggregation of revenue
Project revenue	6.5	7.6
Ancillary and transmission services | Natural Gas
Disaggregation of revenue
Project revenue	3.1	4.7
Ancillary and transmission services | Hydroelectric
Disaggregation of revenue
Project revenue	3.4	2.9
Waste heat revenue
Disaggregation of revenue
Project revenue	1.0	0.2
Waste heat revenue | Solid Fuel
Disaggregation of revenue
Project revenue	1.0	0.2
Steam energy and capacity revenue
Disaggregation of revenue
Project revenue	10.5	11.7
Steam energy and capacity revenue | Natural Gas
Disaggregation of revenue
Project revenue	10.5	11.7
Miscellaneous
Disaggregation of revenue
Project revenue	1.3	(2.3)
Miscellaneous | Solid Fuel
Disaggregation of revenue
Project revenue	0.1
Miscellaneous | Natural Gas
Disaggregation of revenue
Project revenue	$ 1.2	$ (2.3)