Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 270,719	$ 261,817	$ 654,939	$ 603,319
Cost of sales	55,744	32,190	171,479	128,267
Gross margin	214,975	229,627	483,460	475,052
Operating, general and administrative	80,826	73,834	161,918	148,179
Property and other taxes	44,310	43,042	89,099	85,855
Depreciation and depletion	41,016	43,541	86,600	87,296
Operating income (loss)	48,823	69,210	145,843	153,722
Interest expense	(23,511)	(23,197)	(47,301)	(46,167)
Other (expense) income	124	876	1,273	(253)
Income tax expense	22,226	(3,102)	20,653	(5,016)
Net Income	47,662	43,787	120,468	102,286
Total assets	5,720,435	5,438,974	5,720,435	5,438,974	$ 5,644,376
Capital expenditures	81,450	64,451	147,027	116,456
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	219,659	209,755	492,696	448,097
Cost of sales	42,661	19,613	119,655	76,886
Gross margin	176,998	190,142	373,041	371,211
Operating, general and administrative	59,328	52,894	117,111	107,542
Property and other taxes	34,834	33,880	69,881	67,373
Depreciation and depletion	33,720	36,139	71,771	72,292
Operating income (loss)	49,116	67,229	114,278	124,004
Interest expense	(19,285)	(20,318)	(38,820)	(39,838)
Other (expense) income	(220)	(52)	(781)	438
Income tax expense	(1,713)	(2,649)	(3,522)	(3,147)
Net Income	27,898	44,210	71,155	81,457
Total assets	4,568,469	4,351,359	4,568,469	4,351,359
Capital expenditures	63,785	52,844	116,092	95,742
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	51,060	52,062	162,243	155,222
Cost of sales	13,083	12,577	51,824	51,381
Gross margin	37,977	39,485	110,419	103,841
Operating, general and administrative	20,965	19,650	41,973	40,869
Property and other taxes	9,474	9,160	19,214	18,478
Depreciation and depletion	7,296	7,394	14,829	14,988
Operating income (loss)	242	3,281	34,403	29,506
Interest expense	(1,501)	(1,161)	(3,011)	(3,015)
Other (expense) income	(53)	(191)	(530)	(83)
Income tax expense	(354)	492	725	(1,734)
Net Income	(1,666)	2,421	31,587	24,674
Total assets	1,147,330	1,072,173	1,147,330	1,072,173
Capital expenditures	17,665	11,607	30,935	20,714
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	533	1,290	2,834	(232)
Property and other taxes	2	2	4	4
Depreciation and depletion	0	8	0	16
Operating income (loss)	(535)	(1,300)	(2,838)	212
Interest expense	(2,725)	(1,718)	(5,470)	(3,314)
Other (expense) income	397	1,119	2,584	(608)
Income tax expense	24,293	(945)	23,450	(135)
Net Income	21,430	(2,844)	17,726	(3,845)
Total assets	4,636	15,442	4,636	15,442
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	0	0	0	0
Other (expense) income	0	0	0	0
Income tax expense	0	0	0	0
Net Income	0	0	0	0
Total assets	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0