Equity method investments in unconsolidated affiliates (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Operating results
Revenue	$ 30.6	$ 33.0	$ 63.7	$ 70.8
Project expenses	22.5	23.9	44.5	50.6
Project other expense	(0.5)	(0.5)	(0.9)	(0.9)
Project income (loss)	7.6	8.6	18.3	19.3
Chambers
Operating results
Revenue	10.4	10.9	23.1	26.3
Project expenses	9.7	9.6	18.5	20.9
Project other expense	(0.5)	(0.5)	(0.9)	(0.9)
Project income (loss)	0.2	0.8	3.7	4.5
Frederickson
Operating results
Revenue	4.8	5.4	9.9	10.1
Project expenses	4.9	4.9	9.4	9.0
Project income (loss)	(0.1)	0.5	0.5	1.1
Orlando
Operating results
Revenue	13.2	14.1	26.7	26.9
Project expenses	6.0	6.7	12.6	13.3
Project income (loss)	7.2	7.4	14.1	13.6
Other
Operating results
Revenue	2.2	2.6	4.0	7.5
Project expenses	1.9	2.7	$ 4.0	7.4
Project income (loss)	$ 0.3	$ (0.1)		$ 0.1
Other | Maximum
Operating results
Percentage of ownership interest	10.00%		10.00%