Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 384,220	$ 341,502
Cost of sales	115,735	96,077
Gross margin	268,485	245,425
Operating, general and administrative	81,092	74,345
Property and other taxes	44,789	42,813
Depreciation and depletion	45,584	43,755
Operating income (loss)	97,020	84,512
Interest expense	(23,790)	(22,970)
Other (expense) income	1,149	(1,129)
Income tax (expense) benefit	(1,573)	(1,914)
Net Income	72,806	58,499
Total assets	5,700,655	5,400,418	$ 5,644,376
Capital expenditures	65,577	52,005
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	273,037	238,342
Cost of sales	76,994	57,273
Gross margin	196,043	181,069
Operating, general and administrative	57,783	54,648
Property and other taxes	35,047	33,493
Depreciation and depletion	38,051	36,153
Operating income (loss)	65,162	56,775
Interest expense	(19,535)	(19,520)
Other (expense) income	(561)	490
Income tax (expense) benefit	(1,809)	(498)
Net Income	43,257	37,247
Total assets	4,544,043	4,319,798
Capital expenditures	52,307	42,898
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	111,183	103,160
Cost of sales	38,741	38,804
Gross margin	72,442	64,356
Operating, general and administrative	21,008	21,219
Property and other taxes	9,740	9,318
Depreciation and depletion	7,533	7,594
Operating income (loss)	34,161	26,225
Interest expense	(1,510)	(1,854)
Other (expense) income	(477)	108
Income tax (expense) benefit	1,079	(2,226)
Net Income	33,253	22,253
Total assets	1,151,929	1,065,103
Capital expenditures	13,270	9,107
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	2,301	(1,522)
Property and other taxes	2	2
Depreciation and depletion	0	8
Operating income (loss)	(2,303)	1,512
Interest expense	(2,745)	(1,596)
Other (expense) income	2,187	(1,727)
Income tax (expense) benefit	(843)	810
Net Income	(3,704)	(1,001)
Total assets	4,683	15,517
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 (expense) income	0	0
Income tax (expense) benefit	0	0
Net Income	0	0
Total assets	0	0
Capital expenditures	$ 0	$ 0