Segment and geographic information - Geographic (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Revenue and Assets
Project revenues	$ 62.3	$ 71.3	$ 135.0	$ 144.3
Property, Plant and Equipment, net of accumulated depreciation	493.6		493.6		$ 502.1
PPA'S and other intangible assets, net of accumulated amortization	131.3		131.3		144.3
Assets	848.9	1,002.6	848.9	1,002.6	935.6
United States
Revenue and Assets
Project revenues	45.0	52.5	93.5	104.6
Property, Plant and Equipment, net of accumulated depreciation	357.3		357.3		353.9
PPA'S and other intangible assets, net of accumulated amortization	130.2		130.2		142.8
Assets	676.9		676.9		762.3
Canada
Revenue and Assets
Project revenues	17.3	$ 18.8	41.5	$ 39.7
Property, Plant and Equipment, net of accumulated depreciation	136.3		136.3		148.2
PPA'S and other intangible assets, net of accumulated amortization	1.1		1.1		1.5
Assets	$ 172.0		$ 172.0		$ 173.3