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Note 13 - Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Total Gross Plant | $ 2,645,478 | $ 2,429,467
Less Accumulated Depreciation and Amortization | 891,684 | 848,369
Net Plant | 1,753,794 | 1,581,098
Electric Plant [Member] |  |
Total Gross Plant | 2,390,468 | 2,189,811
Less Accumulated Depreciation and Amortization | 731,110 | 699,642
Net Plant | 1,659,358 | 1,490,169
Nonelectric Operations Plant [Member] |  |
Total Gross Plant | 255,010 | 239,656
Less Accumulated Depreciation and Amortization | 160,574 | 148,727
Net Plant | 94,436 | 90,929
Electric Generation Equipment [Member] | Electric Plant [Member] |  |
Total Gross Plant | 915,996 | 905,224
Transmission Plant [Member] | Electric Plant [Member] |  |
Total Gross Plant | 647,474 | 512,832
Distribution Plant [Member] | Electric Plant [Member] |  |
Total Gross Plant | 526,146 | 502,261
General Plant [Member] | Electric Plant [Member] |  |
Total Gross Plant | 123,268 | 99,404
Electric Plant in Service [Member] | Electric Plant [Member] |  |
Total Gross Plant | 2,212,884 | 2,019,721
Construction in Progress [Member] | Electric Plant [Member] |  |
Total Gross Plant | 177,584 | 170,090
Construction in Progress [Member] | Nonelectric Plant [Member] |  |
Total Gross Plant | 7,654 | 11,536
Equipment [Member] | Nonelectric Plant [Member] |  |
Total Gross Plant | 187,904 | 170,634
Buildings and Leasehold Improvements [Member] | Nonelectric Plant [Member] |  |
Total Gross Plant | 53,412 | 53,011
Land [Member] | Nonelectric Plant [Member] |  |
Total Gross Plant | 6,040 | 4,475
Nonelectric Operations Plant [Member] | Nonelectric Plant [Member] |  |
Total Gross Plant | $ 247,356 | $ 228,120

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