Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating revenues	$ 269,360	$ 270,719	$ 604,615	$ 654,939
Cost of sales	61,043	55,744	152,315	171,479
Gross margin	208,317	214,975	452,300	483,460
Operating, general and administrative	71,715	80,826	150,720	161,918
Property and other taxes	46,981	44,310	91,480	89,099
Depreciation and depletion	44,782	41,016	90,047	86,600
Operating income	44,839	48,823	120,053	145,843
Interest expense, net	(24,287)	(23,511)	(48,621)	(47,301)
Other income (expense), net	224	124	(1,758)	1,273
Income Tax Benefit	718	22,226	2,524	20,653
Net Income	21,494	47,662	72,198	120,468
Total assets	5,971,775	5,720,435	5,971,775	5,720,435	$ 5,910,702
Capital expenditures	98,111	81,450	176,482	147,027
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	217,938	219,659	462,563	492,696
Cost of sales	48,305	42,661	112,139	119,655
Gross margin	169,633	176,998	350,424	373,041
Operating, general and administrative	53,599	59,328	112,487	117,111
Property and other taxes	36,811	34,834	71,547	69,881
Depreciation and depletion	36,846	33,720	74,022	71,771
Operating income	42,377	49,116	92,368	114,278
Interest expense, net	(21,483)	(19,285)	(42,299)	(38,820)
Other income (expense), net	959	(220)	1,572	(781)
Income Tax Benefit	756	(1,713)	1,412	(3,522)
Net Income	22,609	27,898	53,053	71,155
Total assets	4,353,894	4,568,469	4,353,894	4,568,469
Capital expenditures	79,744	63,785	143,092	116,092
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	51,422	51,060	142,052	162,243
Cost of sales	12,738	13,083	40,176	51,824
Gross margin	38,684	37,977	101,876	110,419
Operating, general and administrative	18,988	20,965	41,289	41,973
Property and other taxes	10,167	9,474	19,928	19,214
Depreciation and depletion	7,936	7,296	16,025	14,829
Operating income	1,593	242	24,634	34,403
Interest expense, net	(1,646)	(1,501)	(3,250)	(3,011)
Other income (expense), net	309	(53)	404	(530)
Income Tax Benefit	(337)	(354)	(1,074)	725
Net Income	(81)	(1,666)	20,714	31,587
Total assets	1,613,313	1,147,330	1,613,313	1,147,330
Capital expenditures	18,367	17,665	33,390	30,935
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	(872)	533	(3,056)	2,834
Property and other taxes	3	2	5	4
Depreciation and depletion	0	0	0	0
Operating income	869	(535)	3,051	(2,838)
Interest expense, net	(1,158)	(2,725)	(3,072)	(5,470)
Other income (expense), net	(1,044)	397	(3,734)	2,584
Income Tax Benefit	299	24,293	2,186	23,450
Net Income	(1,034)	21,430	(1,569)	17,726
Total assets	4,568	4,636	4,568	4,636
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	0	0	0	0
Interest expense, net	0	0	0	0
Other income (expense), net	0	0	0	0
Income Tax Benefit	0	0	0	0
Net Income	0	0	0	0
Total assets	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0