Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Revenues:
Total	$ 919,503	$ 916,447	$ 849,350
Operating Expenses
Production Fuel – Electric	59,256	66,815	59,690
Electric Operation and Maintenance Expenses	153,529	155,534	146,914
Cost of Products Sold (depreciation included below)	355,119	354,559	316,562
Other Nonelectric Expenses	50,782	51,544	41,492
Depreciation and Amortization	78,086	74,666	72,545
Property Taxes – Electric	15,785	15,585	15,053
Total Operating Expenses	784,623	787,058	717,063
Operating Income	134,880	129,389	132,287
Interest Charges	31,411	30,408	29,604
Nonservice Cost Components of Postretirement Benefits	4,293	5,509	5,620
Other Income	5,112	3,461	2,632
Income Before Income Taxes	104,288	96,933	99,695
Income Tax Expense	17,441	14,588	27,256
Net Income	$ 86,847	$ 82,345	$ 72,439
Average Number of Common Shares Outstanding–Basic (in shares)	39,720,847	39,599,944	39,457,261
Average Number of Common Shares Outstanding–Diluted (in shares)	39,953,826	39,892,196	39,748,347
Basic Earnings Per Common Share (in dollars per share)	$ 2.19	$ 2.08	$ 1.84
Diluted Earnings Per Common Share (in dollars per share)	$ 2.17	$ 2.06	$ 1.82
Electricity [Member]
Operating Revenues:
Revenues from Contracts with Customers	$ 458,016	$ 450,637	$ 436,477
Changes in Accrued Revenues under Alternative Revenue Programs	1,032	(439)	(1,971)
Total Electric	459,048	450,198	434,506
Product [Member]
Operating Revenues:
Revenues from Contracts with Customers	460,455	466,249	414,844
Electricity, Purchased [Member]
Operating Expenses
Purchased Power – Electric System Use	$ 72,066	$ 68,355	$ 64,807