Segment and geographic information - Geographic (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Revenue and Assets
Project revenues	$ 66.2	$ 124.0	$ 146.2	$ 222.4
Property, Plant and Equipment, net of accumulated depreciation	573.1		573.1		$ 602.3
United States
Revenue and Assets
Project revenues	47.5	68.6	103.0	128.3
Property, Plant and Equipment, net of accumulated depreciation	409.8		409.8		426.2
Canada
Revenue and Assets
Project revenues	18.7	$ 55.4	43.2	$ 94.1
Property, Plant and Equipment, net of accumulated depreciation	$ 163.3		$ 163.3		$ 176.1