Business Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Financed Capital Expenditures			$ 5,600	$ 8,500
Income Tax Expense (Benefit)	$ 13,088	$ 10,572	28,252	23,548
Net Income (Loss) Attributable to Parent	48,506	42,072	105,204	97,621
UTILITY OPERATING REVENUES	398,456	385,149	1,121,634	1,099,331
Depreciation and amortization	60,373	57,880	179,939	172,492
Interest Expense	30,620	22,917	91,393	70,465
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	61,594	52,644	133,456	121,169
Property, Plant and Equipment, Additions			131,682	170,299
Assets	4,968,308		4,968,308		$ 4,862,053
Cash	11,800		11,800		6,400
Other Long-term Investments	2,700		2,700		4,000
Interest Rate Cash Flow Hedge Liability at Fair Value	39,909		$ 39,909		0
Nonutility assets representation rate, percentage			1.00%
Electric [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	60,373	57,880	$ 179,939	172,492
Interest Expense	22,244	15,255	66,757	47,461
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	69,881	60,347	158,153	144,416
Property, Plant and Equipment, Additions			131,682	170,299
Assets	4,952,080		4,952,080		4,851,712
Other Segments [Member]
Segment Reporting Information [Line Items]
UTILITY OPERATING REVENUES	0	0	0	0
Depreciation and amortization	0	0	0	0
Interest Expense	8,376	7,662	24,636	23,004
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(8,287)	(7,703)	(24,697)	(23,247)
Property, Plant and Equipment, Additions			0	0
Assets	16,228		16,228		$ 10,341
Electricity [Member]
Segment Reporting Information [Line Items]
UTILITY OPERATING REVENUES	$ 398,456	$ 385,149	$ 1,121,634	$ 1,099,331