Nature of business (Details) $ in Millions	1 Months Ended	6 Months Ended
	Feb. 28, 2018 Plant MW	Jun. 30, 2018 USD ($) state project item MW	Jun. 30, 2017 USD ($)
Nature of business
Number of states in which power generation projects operate | state		9
Number of provinces in which power generation projects operate | item		2
Number of power generation projects | project		22
Gross generating capacity of project (in MW)		1,793
Ownership interest in power generation projects (in MW)		1,440
Number of projects which are majority owned | project		18
Number of project which are in operation | project		16
Number of nonoperational power generation projects | project		3
Recent accounting pronouncements adopted
Decrease in cash provided by investing activities | $		$ (2.4)	$ (4.2)
Plant in service
Nature of business
Gross generating capacity of project (in MW)		1,561
Ownership interest in power generation projects (in MW)		1,208
Plant Not In Service Due To No Contract
Nature of business
Number of nonoperational power generation projects | project		2
Plant With Future Commencement Date
Nature of business
Contract term		15 years
ASU 2016-18 | Restatement Adjustment
Recent accounting pronouncements adopted
Decrease in cash provided by investing activities | $			$ (0.8)
San Diego Projects
Nature of business
Gross generating capacity of project (in MW)	112
Ownership interest in power generation projects (in MW)	112
Number of plants ceasing operations | Plant	3