Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 300,998	$ 272,739	$ 926,657	$ 889,310
Cost of sales	96,156	73,577	293,283	265,495
Gross margin	204,842	199,162	633,374	623,815
Operating, general and administrative	68,290	79,296	220,730	222,139
Property and other taxes	40,673	35,712	111,302	100,953
Depreciation and depletion	39,763	35,693	119,551	107,239
Operating income (loss)	56,116	48,461	181,791	193,484
Interest expense	(21,049)	(22,043)	(71,979)	(68,101)
Other income (loss)	(121)	3,769	4,176	5,429
Income Tax Benefit (Expense)	9,659	(6,389)	4,240	(24,616)
Net Income	44,605	23,798	118,228	106,196
Total assets	5,393,941	5,235,078	5,393,941	5,235,078	$ 5,264,695
Capital expenditures	82,752	72,154	203,998	203,324
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	266,629	238,513	756,374	695,921
Cost of sales	89,681	66,197	245,470	196,034
Gross margin	176,948	172,316	510,904	499,887
Operating, general and administrative	50,460	58,298	157,471	179,191
Property and other taxes	32,343	28,648	87,094	78,987
Depreciation and depletion	32,549	28,476	97,614	85,523
Operating income (loss)	61,596	56,894	168,725	156,186
Interest expense	(19,099)	(19,078)	(65,273)	(58,524)
Other income (loss)	982	1,832	2,136	4,773
Income Tax Benefit (Expense)	7,946	(6,553)	3,600	(16,364)
Net Income	51,425	33,095	109,188	86,071
Total assets	4,294,549	4,169,423	4,294,549	4,169,423
Capital expenditures	66,322	57,813	165,885	171,800
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	34,369	34,226	170,283	193,389
Cost of sales	6,475	7,380	47,813	69,461
Gross margin	27,894	26,846	122,470	123,928
Operating, general and administrative	19,141	19,843	61,638	63,554
Property and other taxes	8,328	7,062	24,200	21,958
Depreciation and depletion	7,206	7,209	21,913	21,691
Operating income (loss)	(6,781)	(7,268)	14,719	16,725
Interest expense	(1,249)	(2,562)	(5,018)	(8,304)
Other income (loss)	345	507	925	1,349
Income Tax Benefit (Expense)	1,169	1,883	(574)	(1,621)
Net Income	(6,516)	(7,440)	10,052	8,149
Total assets	1,093,333	1,057,919	1,093,333	1,057,919
Capital expenditures	16,430	14,341	38,113	31,524
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	(1,311)	1,155	1,621	(20,606)
Property and other taxes	2	2	8	8
Depreciation and depletion	8	8	24	25
Operating income (loss)	1,301	(1,165)	(1,653)	20,573
Interest expense	(701)	(403)	(1,688)	(1,273)
Other income (loss)	(1,448)	1,430	1,115	(693)
Income Tax Benefit (Expense)	544	(1,719)	1,214	(6,631)
Net Income	(304)	(1,857)	(1,012)	11,976
Total assets	6,059	7,736	6,059	7,736
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 Benefit (Expense)	0	0	0	0
Net Income	0	0	0	0
Total assets	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0