Equity method investments in unconsolidated affiliates - Op Results (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating results
Revenue from Contract with Customer, Excluding Assessed Tax	$ 272.0	$ 281.6
Net income (loss)	73.5	(43.8)
Equity in earnings of unconsolidated affiliates	42.9	(3.0)
Frederickson
Operating results
Equity in earnings of unconsolidated affiliates	8.3	9.1
Orlando Cogen, LP
Operating results
Equity in earnings of unconsolidated affiliates	33.1	33.0
Chambers Cogen, LP
Operating results
Equity in earnings of unconsolidated affiliates	4.4	(46.0)
Craven County Wood Energy, LP
Operating results
Equity in earnings of unconsolidated affiliates	(1.8)	0.1
Grayling Generating Station, LP
Operating results
Equity in earnings of unconsolidated affiliates	(1.1)	0.8
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Operating results
Revenue from Contract with Customer, Excluding Assessed Tax	140.9	144.0
Project expenses	96.3	96.5
Project other expenses	(1.7)	(50.5)
Equity in earnings of unconsolidated affiliates	42.9	(3.0)
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Frederickson
Operating results
Revenue from Contract with Customer, Excluding Assessed Tax	29.2	36.0
Project expenses	20.9	26.9
Net income (loss)	8.3	9.1
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Orlando Cogen, LP
Operating results
Revenue from Contract with Customer, Excluding Assessed Tax	60.2	61.5
Project expenses	27.0	28.5
Project other expenses	(0.1)
Net income (loss)	33.1	33.0
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Chambers Cogen, LP
Operating results
Revenue from Contract with Customer, Excluding Assessed Tax	38.4	39.4
Project expenses	32.5	34.6
Project other expenses	(1.5)	(50.9)
Net income (loss)	4.4	(46.1)
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Craven County Wood Energy, LP
Operating results
Revenue from Contract with Customer, Excluding Assessed Tax	9.6	4.9
Project expenses	11.4	4.7
Net income (loss)	(1.8)	0.2
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Grayling Generating Station, LP
Operating results
Revenue from Contract with Customer, Excluding Assessed Tax	3.5	2.2
Project expenses	4.5	1.8
Project other expenses	(0.1)	0.4
Net income (loss)	$ (1.1)	$ 0.8