Business Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Income Tax Expense (Benefit)	$ 10,035	$ 9,772
Net Income (Loss) Attributable to Parent	38,784	35,983
UTILITY OPERATING REVENUES	362,201	357,382
Depreciation and amortization	63,089	60,708
Interest Expense	30,067	30,081
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	48,819	45,755
Property, Plant and Equipment, Additions	62,792	54,490
Assets	4,985,255	4,969,919	$ 4,969,919
Interest Rate Cash Flow Hedge Liability at Fair Value	23,511
Electric [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	63,089	60,708
Interest Expense	21,521	21,920
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	57,295	54,967
Property, Plant and Equipment, Additions	62,792	54,490
Assets	4,966,065	4,952,408
Other Segments [Member]
Segment Reporting Information [Line Items]
UTILITY OPERATING REVENUES	0	0
Depreciation and amortization	0	0
Interest Expense	8,546	8,161
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(8,476)	(9,212)
Property, Plant and Equipment, Additions	0	0
Assets	19,190	17,511
Electricity [Member]
Segment Reporting Information [Line Items]
UTILITY OPERATING REVENUES	$ 362,201	$ 357,382