Summary of significant accounting policies (Details 2)	12 Months Ended
	Dec. 31, 2016
Minimum
Depreciation
Estimated useful life under production plant (in years)	20 years
Estimated useful life under transmission and distribution plant (in years)	25 years
Estimated useful life under general plant (in years)	5 years
Maximum
Depreciation
Estimated useful life under production plant (in years)	88 years
Estimated useful life under transmission and distribution plant (in years)	65 years
Estimated useful life under general plant (in years)	65 years