Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating revenues	$ 280,610	$ 274,836	$ 885,225	$ 929,775
Cost of sales	68,038	64,227	220,353	235,706
Gross margin	212,572	210,609	664,872	694,069
Operating, general and administrative	73,322	76,998	224,042	238,916
Property and other taxes	45,306	44,089	136,786	133,188
Depreciation and depletion	44,289	43,166	134,336	129,766
Operating income (loss)	49,655	46,356	169,708	192,199
Interest expense, net	(23,677)	(23,722)	(72,298)	(71,023)
Other income (expense), net	785	(409)	(973)	864
Income Tax Benefit (Expense)	2,703	(555)	5,227	20,098
Net Income	29,466	21,670	101,664	142,138
Total assets	6,075,639	5,810,180	6,075,639	5,810,180	$ 5,910,702
Capital expenditures	106,505	95,847	282,987	242,874
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	244,155	241,237	706,718	733,933
Cost of sales	61,155	58,768	173,294	178,423
Gross margin	183,000	182,469	533,424	555,510
Operating, general and administrative	54,367	57,433	166,854	174,544
Property and other taxes	35,532	34,731	107,079	104,612
Depreciation and depletion	36,670	35,824	110,692	107,595
Operating income (loss)	56,431	54,481	148,799	168,759
Interest expense, net	(21,286)	(19,481)	(63,585)	(58,301)
Other income (expense), net	1,559	(677)	3,131	(1,458)
Income Tax Benefit (Expense)	1,197	(1,415)	2,609	(4,937)
Net Income	37,901	32,908	90,954	104,063
Total assets	4,430,279	4,632,077	4,430,279	4,632,077
Capital expenditures	87,432	70,063	230,524	186,155
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	36,455	33,599	178,507	195,842
Cost of sales	6,883	5,459	47,059	57,283
Gross margin	29,572	28,140	131,448	138,559
Operating, general and administrative	20,059	18,830	61,348	60,803
Property and other taxes	9,772	9,355	29,700	28,569
Depreciation and depletion	7,619	7,342	23,644	22,171
Operating income (loss)	(7,878)	(7,387)	16,756	27,016
Interest expense, net	(1,574)	(1,588)	(4,824)	(4,599)
Other income (expense), net	459	(344)	863	(874)
Income Tax Benefit (Expense)	607	(232)	(467)	493
Net Income	(8,386)	(9,551)	12,328	22,036
Total assets	1,634,606	1,173,513	1,634,606	1,173,513
Capital expenditures	19,073	25,784	52,463	56,719
All Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Cost of sales	0	0	0	0
Gross margin	0	0	0	0
Operating, general and administrative	(1,104)	735	(4,160)	3,569
Property and other taxes	2	3	7	7
Depreciation and depletion	0	0	0	0
Operating income (loss)	1,102	(738)	4,153	(3,576)
Interest expense, net	(817)	(2,653)	(3,889)	(8,123)
Other income (expense), net	(1,233)	612	(4,967)	3,196
Income Tax Benefit (Expense)	899	1,092	3,085	24,542
Net Income	(49)	(1,687)	(1,618)	16,039
Total assets	10,754	4,590	10,754	4,590
Capital expenditures	0	0	0	0
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Cost of sales	0	0	0	0
Gross margin	0	0	0	0
Operating, general and administrative	0	0	0	0
Property and other taxes	0	0	0	0
Depreciation and depletion	0	0	0	0
Operating income (loss)	0	0	0	0
Interest expense, net	0	0	0	0
Other income (expense), net	0	0	0	0
Income Tax Benefit (Expense)	0	0	0	0
Net Income	0	0	0	0
Total assets	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0