Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 332,539	$ 346,011
Cost of sales	115,434	112,391
Gross margin	217,105	233,620
Operating, general and administrative	79,861	81,123
Property and other taxes	35,421	32,787
Depreciation and depletion	39,890	35,819
Operating income (loss)	61,933	83,891
Interest expense	(24,509)	(23,115)
Other income	3,102	665
Income tax (expense) benefit	(2,472)	(10,016)
Net Income	38,054	51,425
Total assets	5,261,173	4,975,429	$ 5,264,695
Capital expenditures	51,318	56,538
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	241,342	236,046
Cost of sales	83,624	63,919
Gross margin	157,718	172,127
Operating, general and administrative	55,443	60,055
Property and other taxes	27,429	25,259
Depreciation and depletion	32,521	28,554
Operating income (loss)	42,325	58,259
Interest expense	(22,055)	(19,698)
Other income	467	1,282
Income tax (expense) benefit	(1,015)	(6,253)
Net Income	19,722	33,590
Total assets	4,180,672	3,919,536
Capital expenditures	41,625	50,061
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	91,197	109,965
Cost of sales	31,810	48,472
Gross margin	59,387	61,493
Operating, general and administrative	21,912	21,911
Property and other taxes	7,989	7,525
Depreciation and depletion	7,361	7,256
Operating income (loss)	22,125	24,801
Interest expense	(1,955)	(2,994)
Other income	309	142
Income tax (expense) benefit	(3,021)	(4,721)
Net Income	17,458	17,228
Total assets	1,073,778	1,048,449
Capital expenditures	9,693	6,477
All Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0
Cost of sales	0	0
Gross margin	0	0
Operating, general and administrative	2,506	(843)
Property and other taxes	3	3
Depreciation and depletion	8	9
Operating income (loss)	(2,517)	831
Interest expense	(499)	(423)
Other income	2,326	(759)
Income tax (expense) benefit	1,564	958
Net Income	874	607
Total assets	6,723	7,444
Capital expenditures	0	0
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0
Cost of sales	0	0
Gross margin	0	0
Operating, general and administrative	0	0
Property and other taxes	0	0
Depreciation and depletion	0	0
Operating income (loss)	0	0
Interest expense	0	0
Other income	0	0
Income tax (expense) benefit	0	0
Net Income	0	0
Total assets	0	0
Capital expenditures	$ 0	$ 0