Equity method investments in unconsolidated affiliates - Op Results (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Operating results
Revenue	$ 62.3	$ 71.3	$ 135.0	$ 144.3
Net income (loss)	(4.0)	2.9	19.7	5.3
Equity in earnings of unconsolidated affiliates	6.1	9.4	19.8	22.3
Distributions from equity method investments			(21.6)	(25.4)
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Operating results
Revenue	30.5	34.6	68.3	67.4
Project expenses	24.0	24.7	47.7	44.2
Project other expenses	(0.4)	(0.5)	(0.8)	(0.9)
Equity in earnings of unconsolidated affiliates	6.1	9.4	19.8	22.3
Distributions from equity method investments	(15.5)	(19.6)	(21.6)	(25.4)
Deficit in earnings of equity method investments, net of distributions	(9.4)	(10.2)	(1.8)	(3.1)
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Frederickson
Operating results
Revenue	6.1	10.2	14.2	16.1
Project expenses	5.0	9.0	10.5	12.4
Net income (loss)	1.1	1.2	3.7	3.7
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Orlando Cogen, LP
Operating results
Revenue	14.6	15.3	29.1	30.7
Project expenses	6.8	7.1	13.1	14.6
Net income (loss)	7.8	8.2	16.0	16.1
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Chambers Cogen, LP
Operating results
Revenue	9.0	9.1	19.6	20.6
Project expenses	7.9	8.6	16.0	17.2
Project other expenses	(0.4)	$ (0.5)	(0.8)	(0.9)
Net income (loss)	0.7		2.8	$ 2.5
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Craven County Wood Energy, LP
Operating results
Revenue	0.7		3.8
Project expenses	3.2		5.7
Net income (loss)	(2.5)		(1.9)
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] | Grayling Generating Station, LP
Operating results
Revenue	0.1		1.6
Project expenses	1.1		2.4
Net income (loss)	$ (1.0)		$ (0.8)