Summary of significant accounting policies (Details 2)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Depreciation
Composite annual depreciation rate (as a percent)	3.10%	3.10%	3.10%
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