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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

X

- Definition

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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