Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 293,120	$ 270,560	$ 625,659	$ 616,571
Cost of sales	81,693	79,527	197,127	191,918
Gross margin	211,427	191,033	428,532	424,653
Operating, general and administrative	72,579	61,720	152,440	142,843
Property and other taxes	35,208	32,454	70,629	65,241
Depreciation and depletion	39,898	35,727	79,788	71,546
Operating income (loss)	63,742	61,132	125,675	145,023
Interest expense	(26,421)	(22,943)	(50,930)	(46,058)
Other income	1,195	995	4,297	1,660
Income tax (expense) benefit	(2,947)	(8,211)	(5,419)	(18,227)
Net Income	35,569	30,973	73,623	82,398
Total assets	5,307,606	5,060,068	5,307,606	5,060,068	$ 5,264,695
Capital expenditures	69,928	74,632	121,246	131,170
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	248,403	221,362	489,745	457,408
Cost of sales	72,165	65,918	155,789	129,837
Gross margin	176,238	155,444	333,956	327,571
Operating, general and administrative	51,568	60,838	107,011	120,893
Property and other taxes	27,322	25,080	54,751	50,339
Depreciation and depletion	32,544	28,493	65,065	57,047
Operating income (loss)	64,804	41,033	107,129	99,292
Interest expense	(24,119)	(19,748)	(46,174)	(39,446)
Other income	687	1,659	1,154	2,941
Income tax (expense) benefit	(3,331)	(3,558)	(4,346)	(9,811)
Net Income	38,041	19,386	57,763	52,976
Total assets	4,221,293	3,994,221	4,221,293	3,994,221
Capital expenditures	57,938	63,926	99,563	113,987
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	44,717	49,198	135,914	159,163
Cost of sales	9,528	13,609	41,338	62,081
Gross margin	35,189	35,589	94,576	97,082
Operating, general and administrative	20,585	21,800	42,497	43,711
Property and other taxes	7,883	7,371	15,872	14,896
Depreciation and depletion	7,346	7,226	14,707	14,482
Operating income (loss)	(625)	(808)	21,500	23,993
Interest expense	(1,814)	(2,748)	(3,769)	(5,742)
Other income	271	700	580	842
Income tax (expense) benefit	1,278	1,217	(1,743)	(3,504)
Net Income	(890)	(1,639)	16,568	15,589
Total assets	1,080,065	1,057,958	1,080,065	1,057,958
Capital expenditures	11,990	10,706	21,683	17,183
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	426	(20,918)	2,932	(21,761)
Property and other taxes	3	3	6	6
Depreciation and depletion	8	8	16	17
Operating income (loss)	(437)	20,907	(2,954)	21,738
Interest expense	(488)	(447)	(987)	(870)
Other income	237	(1,364)	2,563	(2,123)
Income tax (expense) benefit	(894)	(5,870)	670	(4,912)
Net Income	(1,582)	13,226	(708)	13,833
Total assets	6,248	7,889	6,248	7,889
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	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