Segment and geographic information - EBITDA (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment and geographic information
Number of reportable segments | segment			4
Project revenues	$ 62.3	$ 71.3	$ 135.0	$ 144.3
Segment assets	848.9	1,002.6	848.9	1,002.6	$ 935.6
Project Adjusted EBITDA	36.7	50.8	87.6	104.5
Change in fair value of derivative instruments	(3.1)	7.0	2.5	9.4
Depreciation and amortization	19.4	20.0	39.3	40.2
Interest, net	0.7	0.8	1.4	1.5
Other project expense		1.3		1.2
Project (loss) income	19.7	21.7	44.4	52.2
Administration	4.5	5.0	11.2	11.8
Interest expense, net	10.2	11.0	21.0	22.1
Foreign exchange loss (gain)	9.3	4.9	(11.3)	9.9
Other income (expenses), net	(1.5)	(3.7)	1.1	0.9
Net income (loss) before income taxes	(2.8)	4.5	22.4	7.5
Income tax expense	1.2	1.6	2.7	2.2
Net income (loss)	(4.0)	2.9	19.7	5.3
Solid Fuel
Segment and geographic information
Project revenues	17.3	18.6	40.6	38.3
Segment assets	217.7	261.2	217.7	261.2
Project Adjusted EBITDA	(1.6)	8.3	6.2	18.4
Depreciation and amortization	5.6	5.9	11.2	11.8
Interest, net	0.7	0.8	1.4	1.5
Project (loss) income	(7.9)	1.6	(6.4)	5.1
Net income (loss) before income taxes	(7.9)	1.6	(6.4)	5.1
Net income (loss)	(7.9)	1.6	(6.4)	5.1
Natural Gas
Segment and geographic information
Project revenues	26.9	30.0	57.7	64.5
Segment assets	218.8	262.1	218.8	262.1
Project Adjusted EBITDA	23.8	22.9	52.0	51.0
Change in fair value of derivative instruments	(2.5)	2.4	(3.0)	2.7
Depreciation and amortization	8.9	9.2	18.2	18.7
Other project expense		1.3		1.2
Project (loss) income	17.4	10.0	36.8	28.4
Net income (loss) before income taxes	17.4	10.0	36.8	28.4
Net income (loss)	17.4	10.0	36.8	28.4
Hydroelectric
Segment and geographic information
Project revenues	17.9	22.5	36.2	41.0
Segment assets	357.6	382.9	357.6	382.9
Project Adjusted EBITDA	14.8	19.3	30.1	34.9
Depreciation and amortization	4.9	4.9	9.8	9.7
Project (loss) income	9.9	14.4	20.3	25.2
Net income (loss) before income taxes	9.9	14.4	20.3	25.2
Net income (loss)	9.9	14.4	20.3	25.2
Corporate
Segment and geographic information
Project revenues	0.2	0.2	0.5	0.5
Segment assets	54.8	96.4	54.8	96.4
Project Adjusted EBITDA	(0.3)	0.3	(0.7)	0.2
Change in fair value of derivative instruments	(0.6)	4.6	5.5	6.7
Depreciation and amortization			0.1
Project (loss) income	0.3	(4.3)	(6.3)	(6.5)
Administration	4.5	5.0	11.2	11.8
Interest expense, net	10.2	11.0	21.0	22.1
Foreign exchange loss (gain)	9.3	4.9	(11.3)	9.9
Other income (expenses), net	(1.5)	(3.7)	1.1	0.9
Net income (loss) before income taxes	(22.2)	(21.5)	(28.3)	(51.2)
Income tax expense	1.2	1.6	2.7	2.2
Net income (loss)	$ (23.4)	$ (23.1)	$ (31.0)	$ (53.4)