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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 |  |

X
- Definition Useful life of assets common to business units owned by public utility, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Property, Plant and Equipment, Common, Useful Life + References No definition available. + Details
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X
- Definition Useful life of utility generation assets owned by public utility, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Useful life of utility transmission and distribution equipment owned by public utility, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Composite depreciation rate for public utility plants in service. + References No definition available. + Details
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