Segment and Related Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 347,341	$ 313,445	$ 1,372,316	$ 1,198,670	$ 1,257,910
Utilities Operating Expense, Fuel Used			425,548	306,190	318,020
Utility Margin			946,768	892,480	939,890
Operating and maintenance			208,303	202,991	209,052
Administrative and general			101,873	94,124	109,177
Property and other taxes			173,444	179,517	171,888
Depreciation and depletion			187,467	179,644	172,923
Operating Income	79,990	66,496	275,681	236,204	276,850
Interest expense, net			(93,674)	(96,812)	(95,068)
Other income, net			8,252	4,853	413
Income tax (expense) benefit			(3,419)	10,970	19,925
Net Income	51,336	53,551	186,840	155,215	202,120
Total assets	(6,780,443)	(6,389,449)	(6,780,443)	(6,389,449)	(6,083,486)
Capital expenditures			434,328	405,762	316,016
Eliminations
Segment Reporting Information [Line Items]
Total revenues			0	0	0
Utilities Operating Expense, Fuel Used			0	0	0
Utility Margin			0	0	0
Operating and maintenance			0	0	0
Administrative and general			0	0	0
Property and other taxes			0	0	0
Depreciation and depletion			0	0	0
Operating Income			0	0	0
Interest expense, net			0	0	0
Other income, net			0	0	0
Income tax (expense) benefit			0	0	0
Net Income			0	0	0
Total assets	0	0	0	0	0
Capital expenditures			0	0	0
Operating Segments | Electric
Segment Reporting Information [Line Items]
Total revenues			1,052,182	940,815	981,178
Utilities Operating Expense, Fuel Used			294,820	236,581	239,589
Utility Margin			757,362	704,234	741,589
Operating and maintenance			156,383	149,220	155,285
Administrative and general			72,641	69,602	77,139
Property and other taxes			134,910	140,621	134,686
Depreciation and depletion			154,626	147,968	143,262
Operating Income			238,802	196,823	231,217
Interest expense, net			(82,678)	(85,487)	(78,809)
Other income, net			3,676	4,867	(1,365)
Income tax (expense) benefit			(2,512)	11,282	(6,079)
Net Income			157,288	127,485	144,964
Total assets	(5,432,578)	(5,126,589)	(5,432,578)	(5,126,589)	(4,808,011)
Capital expenditures			354,775	324,369	241,190
Operating Segments | Electric | Revision of Prior Period, Error Correction, Adjustment
Segment Reporting Information [Line Items]
Total assets	(488,300)		(488,300)
Operating Segments | Gas Domestic Regulated [Member]
Segment Reporting Information [Line Items]
Total revenues			320,134	257,855	276,732
Utilities Operating Expense, Fuel Used			130,728	69,609	78,431
Utility Margin			189,406	188,246	198,301
Operating and maintenance			51,920	53,771	53,767
Administrative and general			27,550	26,311	28,965
Property and other taxes			38,526	38,887	37,192
Depreciation and depletion			32,841	31,676	29,661
Operating Income			38,569	37,601	48,716
Interest expense, net			(6,083)	(6,341)	(6,218)
Other income, net			3,046	2,704	(814)
Income tax (expense) benefit			(2,640)	(2,426)	493
Net Income			32,892	31,538	42,177
Total assets	(1,342,031)	(1,251,240)	(1,342,031)	(1,251,240)	(1,270,811)
Capital expenditures			79,553	81,393	74,826
Operating Segments | Gas Domestic Regulated [Member] | Revision of Prior Period, Error Correction, Adjustment
Segment Reporting Information [Line Items]
Total assets	488,300		488,300
Other Segments [Member] | Operating Segments
Segment Reporting Information [Line Items]
Total revenues			0	0	0
Utilities Operating Expense, Fuel Used			0	0	0
Utility Margin			0	0	0
Operating and maintenance			0	0	0
Administrative and general			1,682	(1,789)	3,073
Property and other taxes			8	9	10
Depreciation and depletion			0	0	0
Operating Income			(1,690)	1,780	(3,083)
Interest expense, net			(4,913)	(4,984)	(10,041)
Other income, net			1,530	(2,718)	2,592
Income tax (expense) benefit			1,733	2,114	25,511
Net Income			(3,340)	(3,808)	14,979
Total assets	$ (5,834)	$ (11,620)	(5,834)	(11,620)	(4,664)
Capital expenditures			$ 0	$ 0	$ 0