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Summary of Significant Accounting Policies (Utility Plant) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Capitalized interest costs | $ 2,856 | $ 2,807 | $ 2,188
Construction in progress | 68,765 | 45,851 |
Major components of depreciable plant and equipment [Abstract] |  |  |
Equipment | 335,358 | 307,938 |
Transmission and distribution | 1,375,821 | 1,295,690 |
Office buildings and other structures | 121,872 | 110,600 |
Total depreciable plant and equipment | $ 1,833,051 | $ 1,714,228 |
Estimated Useful Lives of Plant and Equipment [Abstract] |  |  |
Depreciation expense as percentage of beginning of year balance of depreciable plant | 3.60% | 3.60% | 3.50%
Minimum [Member] |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life | 7 years |  |
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 |  |
Estimated service lives of assets | 5 years |  |
Useful life of intangible assets | 5 years |  |
Maximum [Member] |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life | 39 years |  |
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 |  |
Estimated service lives of assets | 75 years |  |
Useful life of intangible assets | 70 years |  |
Administrative and general expense |  |  |
Estimated Useful Lives of Plant and Equipment [Abstract] |  |  |
Depreciation | $ 2,306 | $ 2,209 | $ 1,670
Water Plant [Member] |  |  |
Estimated Useful Lives of Plant and Equipment [Abstract] |  |  |
Depreciation | $ 53,031 | $ 46,456 | $ 42,659
Building and Building Improvements [Member] | Minimum [Member] |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life | 7 years |  |
Building and Building Improvements [Member] | Maximum [Member] |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life | 39 years |  |
Finite-Lived Intangible Assets [Member] | Minimum [Member] |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life | 7 years |  |
Finite-Lived Intangible Assets [Member] | Maximum [Member] |  |  |
Public Utility, Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life | 12 years |  |

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