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Summary of Significant Accounting Policies - Property, Plant and Equipment (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Major components of depreciable plant and equipment [Abstract] |  |  |
Equipment | $ 567,904 | $ 521,183 |
Transmission and distribution | 1,699,953 | 2,207,051 |
Office buildings and other structures | 930,203 | 260,220 |
Total depreciable plant and equipment | $ 3,198,060 | $ 2,988,454 |
Estimated Useful Lives of Plant and Equipment [Abstract] |  |  |
Depreciation expense as percentage of beginning of year balance of depreciable plant | 3.20% | 3.80% | 3.60%
Capitalized interest costs | $ 4,133 | $ 4,323 | $ 2,856
Equity funds capitalized | 1,249 |  |
Administrative and general expense |  |  |
Estimated Useful Lives of Plant and Equipment [Abstract] |  |  |
Depreciation | 2,343 | 2,869 | 2,306
Water Plant |  |  |
Estimated Useful Lives of Plant and Equipment [Abstract] |  |  |
Depreciation | $ 86,823 | $ 63,785 | $ 53,031
Minimum |  |  |
Estimated Useful Lives of Plant and Equipment [Abstract] |  |  |
Estimated useful life of equipment | 5 years |  |
Estimated useful life of transmission and distribution plant | 35 years |  |
Estimated useful lives of office buildings and other structures | 7 years |  |
Useful life of intangible assets | 5 years |  |
Maximum |  |  |
Estimated Useful Lives of Plant and Equipment [Abstract] |  |  |
Estimated useful life of equipment | 35 years |  |
Estimated useful life of transmission and distribution plant | 75 years |  |
Estimated useful lives of office buildings and other structures | 50 years |  |
Useful life of intangible assets | 70 years |  |

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