Nature of business (Details) $ in Millions	6 Months Ended
	Jun. 30, 2016 USD ($) state project item MW	Dec. 31, 2015 USD ($)
Nature of business
Gross generating capacity of project (in MW) | MW	2,138
Ownership interest in operational power generation projects (in MW) | MW	1,500
Number of operational power generation projects | project	23
Number of states in which power generation projects operate | state	11
Number of provinces in which power generation projects operate | item	2
Number of projects which are majority owned | project	18
Accounting Standards Update 201503 (Member) | Restatement Adjustment [Member]
Recent accounting pronouncements adopted
Long-term debt and convertible debentures | $	$ 22.2	$ 42.5