Segment and geographic information (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment and geographic information
Number of reportable segments | segment			4
Project revenues	$ 124.0	$ 98.2	$ 222.4	$ 204.6
Segment assets	1,374.1	1,709.9	1,374.1	1,709.9	$ 1,456.8
Project Adjusted EBITDA	85.4	46.2	149.3	108.7
Change in fair value of derivative instruments	2.6	(12.2)	3.8	(11.0)
Depreciation and amortization	34.7	30.4	69.3	60.3
Interest, net	2.5	2.9	5.3	5.4
Impairment	57.7		57.7
Other project expense		(0.1)		0.1
Project (loss) income	(12.1)	25.2	13.2	53.9
Administration	5.7	5.8	12.1	11.9
Interest expense, net	18.4	51.2	35.7	67.8
Foreign exchange loss	5.9	2.6	8.3	22.5
Other income, net		0.3		(2.2)
Loss from operations before income taxes	(42.1)	(34.7)	(42.9)	(46.1)
Income tax expense (benefit)	(22.3)	(18.4)	(22.6)	(16.8)
Net income (loss) from continuing operations	(19.8)	(16.3)	(20.3)	(29.3)
East U.S.
Segment and geographic information
Project revenues	40.4	33.7	76.5	73.1
Segment assets	688.5	775.7	688.5	775.7
Project Adjusted EBITDA	29.1	20.9	56.2	51.2
Change in fair value of derivative instruments	0.7	(2.5)	1.3	(1.7)
Depreciation and amortization	11.4	10.9	22.7	21.9
Interest, net	2.6	2.9	5.3	5.4
Impairment	57.7		57.7
Project (loss) income	(43.3)	9.6	(30.8)	25.6
Loss from operations before income taxes	(43.3)	9.6	(30.8)	25.6
Net income (loss) from continuing operations	(43.3)	9.6	(30.8)	25.6
West U.S.
Segment and geographic information
Project revenues	27.9	25.5	51.3	44.5
Segment assets	299.3	335.9	299.3	335.9
Project Adjusted EBITDA	10.6	14.5	19.8	22.0
Depreciation and amortization	10.0	9.9	19.8	19.7
Interest, net	(0.1)
Project (loss) income	0.7	4.6		2.3
Loss from operations before income taxes	0.7	4.6		2.3
Net income (loss) from continuing operations	0.7	4.6		2.3
Canada
Segment and geographic information
Project revenues	55.4	38.8	94.1	86.5
Segment assets	281.9	430.3	281.9	430.3
Project Adjusted EBITDA	45.2	10.9	72.8	35.7
Change in fair value of derivative instruments	0.9	(11.6)	4.1	(12.1)
Depreciation and amortization	13.2	9.6	26.4	18.5
Project (loss) income	31.1	12.9	42.3	29.3
Loss from operations before income taxes	31.1	12.9	42.3	29.3
Net income (loss) from continuing operations	31.1	12.9	42.3	29.3
Un-Allocated Corporate
Segment and geographic information
Project revenues	0.3	0.2	0.5	0.5
Segment assets	104.4	168.0	104.4	168.0
Project Adjusted EBITDA	0.5	(0.1)	0.5	(0.2)
Change in fair value of derivative instruments	1.0	1.9	(1.6)	2.8
Depreciation and amortization	0.1		0.4	0.2
Other project expense		(0.1)		0.1
Project (loss) income	(0.6)	(1.9)	1.7	(3.3)
Administration	5.7	5.8	12.1	11.9
Interest expense, net	18.4	51.2	35.7	67.8
Foreign exchange loss	5.9	2.6	8.3	22.5
Other income, net		0.3		(2.2)
Loss from operations before income taxes	(30.6)	(61.8)	(54.4)	(103.3)
Income tax expense (benefit)	(22.3)	(18.4)	(22.6)	(16.8)
Net income (loss) from continuing operations	$ (8.3)	$ (43.4)	$ (31.8)	$ (86.5)