Equity method investments in unconsolidated affiliates (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended		30 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2017
Operating results
Revenue	$ 31.0	$ 34.1	$ 92.7	$ 97.8
Project expenses	21.3	24.1	69.7	68.5
Project other expense	(0.5)	(0.4)	(59.1)	(1.4)
Equity in (loss) earnings of unconsolidated affiliates	9.2	9.6	(36.1)	27.9
Distributions from equity method investments	(13.7)	(13.0)	(30.9)	(36.5)
Deficit in earnings of equity method investments, net of distributions	(4.5)	(3.4)	(67.0)	(8.6)
Frederickson
Operating results
Revenue	5.9	5.8	16.0	15.7
Project expenses	4.9	4.9	16.8	14.3
Project income (loss)	1.0	0.9	(0.8)	1.4
Orlando Cogen, LP
Operating results
Revenue	14.3	13.9	40.4	40.6
Project expenses	7.2	6.9	22.1	19.6
Project income (loss)	7.1	7.0	18.3	21.0
Koma Kulshan Associates
Operating results
Revenue	0.3	0.1	1.4	1.3
Project expenses	0.3	0.4	0.8	0.9
Project income (loss)		(0.3)	0.6	0.4
Chambers Cogen, LP
Operating results
Revenue	10.5	11.2	33.1	34.3
Project expenses	8.9	9.3	27.2	27.8
Project other expense	(0.5)	(0.4)	(48.5)	(1.4)
Project income (loss)	$ 1.1	1.5	(42.6)	5.1
Equity in (loss) earnings of unconsolidated affiliates			3.4		$ 5.5	$ 6.5
Distributions from equity method investments							$ (33.6)
Selkirk Cogen Partners, LP
Operating results
Revenue		3.1	1.8	5.9
Project expenses		2.6	2.8	$ 5.9
Project other expense			(10.6)
Project income (loss)		$ 0.5	(11.6)
Equity in (loss) earnings of unconsolidated affiliates			$ 1.2