Segment and geographic information - EBITDA (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2018 USD ($) item	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment and geographic information
Number of reportable segments			4	4
Project revenues	$ 66.2	$ 124.0			$ 146.2	$ 222.4
Segment assets	1,067.8	1,374.1	$ 1,067.8	$ 1,067.8	1,067.8	1,374.1	$ 1,158.8
Goodwill	21.4		21.4	21.4	21.4		$ 21.3
Project Adjusted EBITDA	39.8	85.4			93.2	149.3
Change in fair value of derivative instruments	0.2	2.6			(3.6)	3.8
Depreciation and amortization	25.1	34.7			53.1	69.3
Interest, net	0.9	2.5			1.9	5.3
Impairment		57.7				57.7
Project income (loss)	13.6	(12.1)			41.8	13.2
Administration	6.2	5.7			12.2	12.1
Interest expense, net	11.1	18.4			26.1	35.7
Foreign exchange (gain) loss	(5.4)	5.9			(13.6)	8.3
Other income, net	(0.2)				(2.2)
Income (loss) from operations before income taxes	1.9	(42.1)			19.3	(42.9)
Income tax benefit	0.9	(22.3)			4.2	(22.6)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	1.0	(19.8)			15.1	(20.3)
Net income (loss)	1.0	(19.8)			15.1	(20.3)
East
Segment and geographic information
Project revenues	39.0	40.4			80.6	76.5
Segment assets	610.5	688.5	610.5	610.5	610.5	688.5
Project Adjusted EBITDA	31.2	29.1			64.4	56.2
Change in fair value of derivative instruments	0.5	0.7			0.2	1.3
Depreciation and amortization	11.4	11.4			23.2	22.7
Interest, net	0.9	2.6			1.9	5.3
Impairment		57.7				57.7
Project income (loss)	18.4	(43.3)			39.1	(30.8)
Income (loss) from operations before income taxes	18.4	(43.3)			39.1	(30.8)
Net income (loss)	18.4	(43.3)			39.1	(30.8)
West
Segment and geographic information
Project revenues	8.3	27.9			21.9	51.3
Segment assets	167.1	299.3	167.1	167.1	167.1	299.3
Project Adjusted EBITDA	(0.7)	10.6			5.4	19.8
Depreciation and amortization	5.6	10.0			13.6	19.8
Interest, net		(0.1)
Project income (loss)	(6.3)	0.7			(8.2)
Income (loss) from operations before income taxes	(6.3)	0.7			(8.2)
Net income (loss)	(6.3)	0.7			(8.2)
Canada
Segment and geographic information
Project revenues	18.7	55.4			43.2	94.1
Segment assets	197.6	281.9	197.6	197.6	197.6	281.9
Project Adjusted EBITDA	9.0	45.2			23.2	72.8
Change in fair value of derivative instruments	(0.2)	0.9			(1.4)	4.1
Depreciation and amortization	8.0	13.2			16.1	26.4
Project income (loss)	1.2	31.1			8.5	42.3
Income (loss) from operations before income taxes	1.2	31.1			8.5	42.3
Net income (loss)	1.2	31.1			8.5	42.3
Un-Allocated Corporate
Segment and geographic information
Project revenues	0.2	0.3			0.5	0.5
Segment assets	92.6	104.4	$ 92.6	$ 92.6	92.6	104.4
Project Adjusted EBITDA	0.3	0.5			0.2	0.5
Change in fair value of derivative instruments	(0.1)	1.0			(2.4)	(1.6)
Depreciation and amortization	0.1	0.1			0.2	0.4
Project income (loss)	0.3	(0.6)			2.4	1.7
Administration	6.2	5.7			12.2	12.1
Interest expense, net	11.1	18.4			26.1	35.7
Foreign exchange (gain) loss	(5.4)	5.9			(13.6)	8.3
Other income, net	(0.2)				(2.2)
Income (loss) from operations before income taxes	(11.4)	(30.6)			(20.1)	(54.4)
Income tax benefit	0.9	(22.3)			4.2	(22.6)
Net income (loss)	$ (12.3)	$ (8.3)			$ (24.3)	$ (31.8)