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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 32
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-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130545-203045

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Revenue from contracts - Disaggregation of revenue (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 segment	Jun. 30, 2018 item	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Disaggregation of revenue
Number of reportable segments			4	4
Total revenue	$ 66.2	$ 124.0			$ 146.2	$ 222.4
Energy sales
Disaggregation of revenue
Project revenue	31.4	40.0			69.8	77.1
Total revenue	31.4	40.0			69.8	77.1
Energy capacity revenue
Disaggregation of revenue
Project revenue	23.3	28.3			43.4	47.8
Total revenue	23.3	28.3			43.4	47.8
Other
Disaggregation of revenue
Misc. revenue		(0.3)			0.1	(0.5)
Total revenue	11.5	55.7			33.0	97.5
Steam energy and capacity revenue
Disaggregation of revenue
Project revenue	0.4	9.7			3.7	21.5
Waste heat revenue
Disaggregation of revenue
Project revenue		0.1			0.1	0.3
Enhanced dispatch contracts
Disaggregation of revenue
Project revenue	6.4	40.8			15.0	65.0
Ancillary and transmission services
Disaggregation of revenue
Project revenue	4.7	5.1			14.1	10.7
Asset management and operation
Disaggregation of revenue
Project revenue		0.3				0.5
East
Disaggregation of revenue
Total revenue	39.0	40.4			80.6	76.5
East \| Energy sales
Disaggregation of revenue
Project revenue	21.6	24.4			47.0	46.2
East \| Energy capacity revenue
Disaggregation of revenue
Project revenue	13.6	12.4			24.9	22.5
East \| Steam energy and capacity revenue
Disaggregation of revenue
Project revenue	0.4	2.6			0.9	5.9
East \| Ancillary and transmission services
Disaggregation of revenue
Project revenue	3.4	1.0			7.8	1.9
West
Disaggregation of revenue
Total revenue	8.3	27.9			21.9	51.3
West \| Energy sales
Disaggregation of revenue
Project revenue	1.7	7.8			6.8	16.2
West \| Energy capacity revenue
Disaggregation of revenue
Project revenue	6.8	13.3			12.7	20.0
West \| Other
Disaggregation of revenue
Misc. revenue	(0.2)	(0.3)			(0.4)	(0.5)
West \| Steam energy and capacity revenue
Disaggregation of revenue
Project revenue		7.1			2.8	15.6
Canada
Disaggregation of revenue
Total revenue	18.7	55.4			43.2	94.1
Canada \| Energy sales
Disaggregation of revenue
Project revenue	8.1	7.8			16.0	14.7
Canada \| Energy capacity revenue
Disaggregation of revenue
Project revenue	2.9	2.6			5.8	5.3
Canada \| Waste heat revenue
Disaggregation of revenue
Project revenue		0.1			0.1	0.3
Canada \| Enhanced dispatch contracts
Disaggregation of revenue
Project revenue	6.4	40.8			15.0	65.0
Canada \| Ancillary and transmission services
Disaggregation of revenue
Project revenue	1.3	4.1			6.3	8.8
Un-Allocated Corporate
Disaggregation of revenue
Total revenue	0.2	0.3			0.5	0.5
Un-Allocated Corporate \| Other
Disaggregation of revenue
Misc. revenue	$ 0.2				$ 0.5
Un-Allocated Corporate \| Asset management and operation
Disaggregation of revenue
Project revenue		$ 0.3				$ 0.5