Equity method investments in unconsolidated affiliates (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended		30 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2017
Operating results
Revenue	$ 29.8	$ 30.6	$ 61.7	$ 63.7
Project expenses	26.1	22.5	48.5	44.5
Project other expense	(58.1)	(0.5)	(58.6)	(0.9)
Equity in (loss) earnings of unconsolidated affiliates	(54.4)	7.6	(45.4)	18.3
Distributions from equity method investments	(13.5)	(19.2)	(17.2)	(23.5)
Surplus (deficit) in earnings of equity method investments, net of distributions	(67.9)	(11.6)	(62.6)	(5.2)
Frederickson
Operating results
Revenue	4.8	4.8	10.1	9.9
Project expenses	7.5	4.9	11.9	9.4
Project income (loss)	(2.7)	(0.1)	(1.8)	0.5
Orlando Cogen, LP
Operating results
Revenue	12.9	13.2	26.1	26.7
Project expenses	7.8	6.0	14.9	12.6
Project income (loss)	5.1	7.2	11.2	14.1
Koma Kulshan Associates
Operating results
Revenue	0.8	0.8	1.1	1.2
Project expenses	0.3	0.2	0.6	0.6
Project income (loss)	0.5	0.6	0.5	0.6
Chambers Cogen, LP
Operating results
Revenue	10.3	10.4	22.6	23.1
Project expenses	9.2	9.7	18.3	18.5
Project other expense	(47.5)	(0.5)	(48.0)	(0.9)
Project income (loss)	(46.4)	0.2	(43.7)	3.7
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	1.0	1.4	1.8	2.8
Project expenses	1.3	1.7	2.8	3.4
Project other expense	(10.6)		(10.6)
Project income (loss)	$ (10.9)	$ (0.3)	(11.6)	$ (0.6)
Equity in (loss) earnings of unconsolidated affiliates			$ 1.2