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Summary of significant accounting policies (Details 2) | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Depreciation |  |  |
Composite annual depreciation rate (as a percent) | 3.20% | 3.50% | 3.80%
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) | 50 years |  |

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