Business Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Income Tax Expense (Benefit)	$ 4,960	$ 5,931		$ 15,164	$ 12,976
Net Income (Loss) Attributable to Parent	14,913	23,246		56,698	55,549
UTILITY OPERATING REVENUES	340,869	359,678		723,178	714,182
Depreciation and amortization	59,897	59,286		119,566	114,612
Interest Expense	30,291	23,780		60,773	47,548
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	19,873	29,177		71,862	68,525
Property, Plant and Equipment, Additions				113,118	101,595
Assets	4,857,797			4,857,797		$ 4,862,053
Cash	9,200			9,200		6,400
Other Long-term Investments	2,600			2,600		4,000
Interest Rate Cash Flow Hedge Liability at Fair Value	23,054			23,054		0
Electric [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	59,897	59,286		119,566	114,612
Interest Expense	22,252	16,138		44,513	32,206
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	28,044	36,905		88,272	84,069
Property, Plant and Equipment, Additions				113,118	101,595
Assets	4,843,952			4,843,952		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	8,039	7,642		16,260	15,342
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(8,171)	(7,728)		(16,410)	(15,544)
Property, Plant and Equipment, Additions				0	0
Assets	13,845			13,845		$ 10,341
Electricity [Member]
Segment Reporting Information [Line Items]
UTILITY OPERATING REVENUES	$ 340,869	$ 359,678	$ 714,182	$ 723,178	$ 714,182