Nature of business (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) state contract project item MW	Sep. 30, 2016 USD ($)
Nature of business
Gross generating capacity of project (in MW) | MW	2,138
Ownership interest in power generation projects (in MW) | MW	1,500
Number of power generation projects	23
Number of states in which power generation projects operate | state	9
Number of provinces in which power generation projects operate | item	2
Number of projects which are majority owned	18
Recent accounting pronouncements adopted
Number of PPA's requiring further analysis | contract	12
Increase (decrease) in cash provided by operating activities | $	$ 137.9	$ 91.9
Increase (decrease) in cash provided by investing activities | $	$ (4.9)	0.8
Plant in service
Nature of business
Gross generating capacity of project (in MW) | MW	1,975
Ownership interest in power generation projects (in MW) | MW	1,337
Number of power generation projects	19
Plant not in service
Nature of business
Number of power generation projects	4
Plant Not In Service Due To Revised Contract [Member]
Nature of business
Number of power generation projects	3
Plant With Future Commencement Date [Member]
Nature of business
Contract term	15 years
ASU 2016-18 | Pro Forma [Member]
Recent accounting pronouncements adopted
Increase (decrease) in cash provided by operating activities | $	$ 0.8	2.6
Increase (decrease) in cash provided by investing activities | $	$ 0.8	$ 2.6