Equity method investments in unconsolidated affiliates (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 18, 2018	Jun. 17, 2018	Nov. 30, 2017
Operating results
Revenue	$ 30.3	$ 29.8	$ 63.3	$ 61.7
Project expenses	18.8	26.1	38.9	48.5
Project other expense	(0.3)	(58.1)	(0.9)	(58.6)
Equity in earnings (loss) of unconsolidated affiliates (Note 4)	11.2	(54.4)	23.5	(45.4)
Distributions from equity method investments	(20.7)	(13.5)	(27.3)	(17.2)
Surplus of earnings of equity method investments, net of distributions	(9.5)	(67.9)	(3.8)	(62.6)
Frederickson
Operating results
Revenue	4.7	4.8	9.8	10.1
Project expenses	3.4	7.5	6.6	11.9
Project income (loss)	1.3	(2.7)	3.2	(1.8)
Orlando Cogen, LP
Operating results
Revenue	15.2	12.9	29.7	26.1
Project expenses	7.6	7.8	14.6	14.9
Project income (loss)	7.6	5.1	15.1	11.2
Koma
Operating results
Revenue	0.8	0.8	1.0	1.1
Project expenses	0.3	0.3	0.6	0.6
Project income (loss)	$ 0.5	0.5	$ 0.4	0.5
Percentage of equity method ownership interest	50.00%		50.00%		50.00%	49.75%
Chambers
Operating results
Revenue	$ 9.6	10.3	$ 22.8	22.6
Project expenses	7.5	9.2	17.1	18.3
Project other expense	(0.3)	(47.5)	(0.9)	(48.0)
Project income (loss)	$ 1.8	(46.4)	$ 4.8	(43.7)
Selkirk Cogen Partners, LP
Operating results
Revenue		1.0		1.8
Project expenses		1.3		2.8
Project other expense		(10.6)		(10.6)
Project income (loss)		$ (10.9)		$ (11.6)
Percentage of equity method ownership interest							17.70%