Business Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Income Tax Expense (Benefit)	$ 10,966	$ 10,981	$ 25,506	$ 25,918
Net Income (Loss) Attributable to Parent	42,047	40,299	98,041	95,405
UTILITY OPERATING REVENUES	380,837	356,088	1,073,625	1,024,973
Depreciation and amortization	64,797	62,359	190,880	184,166
Interest Expense	31,544	32,741	93,810	96,757
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	53,013	51,280	123,547	121,323
Assets	5,067,794	4,969,919	5,067,794	4,969,919	$ 4,969,919
Interest Rate Cash Flow Hedge Liability at Fair Value	44,816		44,816
Electric [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	64,797	62,359	190,880	184,166
Interest Expense	20,609	21,725	63,385	65,527
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	63,918	62,098	153,800	155,835
Assets	5,052,557	4,952,408	5,052,557	4,952,408
Other Segments [Member]
Segment Reporting Information [Line Items]
UTILITY OPERATING REVENUES	0	0	0	0
Depreciation and amortization	0	0	0	0
Interest Expense	10,935	11,016	30,425	31,230
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(10,905)	(10,818)	(30,253)	(34,512)
Assets	15,237	17,511	15,237	17,511
Electricity [Member]
Segment Reporting Information [Line Items]
UTILITY OPERATING REVENUES	$ 380,837	$ 356,088	$ 1,073,625	$ 1,024,973