Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Employee service share-based compensation, recognition of excess tax benefits				$ 1,800
Operating revenues	$ 309,933	$ 300,998	$ 961,104	926,657
Cost of sales	97,507	96,156	301,324	293,283
Gross margin	212,426	204,842	659,780	633,374
Operating, general and administrative	70,244	68,290	226,394	220,730
Property and other taxes	39,111	40,673	118,520	111,302
Depreciation and depletion	41,525	39,763	124,481	119,551
Operating income (loss)	61,546	56,116	190,385	181,791
Interest expense	(23,149)	(21,049)	(69,957)	(71,979)
Other income (loss)	790	(121)	4,413	4,176
Income Tax (Expense) Benefit	(2,775)	9,659	(10,032)	6,053
Net Income	36,412	44,605	114,809	120,041
Total assets	5,628,955	5,393,941	5,628,955	5,393,941	$ 5,499,321
Capital expenditures	77,862	82,752	196,985	203,998
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	274,785	266,629	774,890	756,374
Cost of sales	91,327	89,681	246,858	245,470
Gross margin	183,458	176,948	528,032	510,904
Operating, general and administrative	53,535	50,460	166,240	157,471
Property and other taxes	30,754	32,343	92,824	87,094
Depreciation and depletion	34,127	32,549	102,302	97,614
Operating income (loss)	65,042	61,596	166,666	168,725
Interest expense	(20,644)	(19,099)	(62,745)	(65,273)
Other income (loss)	1,247	982	2,870	2,136
Income Tax (Expense) Benefit	(4,153)	7,946	(7,563)	3,600
Net Income	41,492	51,425	99,228	109,188
Total assets	4,498,807	4,294,549	4,498,807	4,294,549
Capital expenditures	62,799	66,322	159,835	165,885
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	35,148	34,369	186,214	170,283
Cost of sales	6,180	6,475	54,466	47,813
Gross margin	28,968	27,894	131,748	122,470
Operating, general and administrative	19,280	19,141	61,115	61,638
Property and other taxes	8,355	8,328	25,688	24,200
Depreciation and depletion	7,390	7,206	22,155	21,913
Operating income (loss)	(6,057)	(6,781)	22,790	14,719
Interest expense	(1,418)	(1,249)	(4,464)	(5,018)
Other income (loss)	732	345	1,449	925
Income Tax (Expense) Benefit	2,334	1,169	(3,800)	(574)
Net Income	(4,409)	(6,516)	15,975	10,052
Total assets	1,127,464	1,093,333	1,127,464	1,093,333
Capital expenditures	15,063	16,430	37,150	38,113
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	(2,571)	(1,311)	(961)	1,621
Property and other taxes	2	2	8	8
Depreciation and depletion	8	8	24	24
Operating income (loss)	2,561	1,301	929	(1,653)
Interest expense	(1,087)	(701)	(2,748)	(1,688)
Other income (loss)	(1,189)	(1,448)	94	1,115
Income Tax (Expense) Benefit	(956)	544	1,331	3,027
Net Income	(671)	(304)	(394)	801
Total assets	2,684	6,059	2,684	6,059
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 income (loss)	0	0	0	0
Income Tax (Expense) Benefit	0	0	0	0
Net Income	0	0	0	0
Total assets	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0