Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating revenues	$ 335,255	$ 384,220
Cost of sales	91,272	115,735
Gross margin	243,983	268,485
Operating, general and administrative	79,005	81,092
Property and other taxes	44,499	44,789
Depreciation and depletion	45,265	45,584
Operating income	75,214	97,020
Interest expense, net	(24,334)	(23,790)
Other income (expense), net	(1,982)	1,149
Income tax (expense) benefit	1,806	(1,573)
Net Income	50,704	72,806
Total assets	5,966,943	5,700,655	$ 5,910,702
Capital expenditures	78,371	65,577
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	244,625	273,037
Cost of sales	63,834	76,994
Gross margin	180,791	196,043
Operating, general and administrative	58,888	57,783
Property and other taxes	34,736	35,047
Depreciation and depletion	37,176	38,051
Operating income	49,991	65,162
Interest expense, net	(20,816)	(19,535)
Other income (expense), net	613	(561)
Income tax (expense) benefit	656	(1,809)
Net Income	30,444	43,257
Total assets	4,345,943	4,544,043
Capital expenditures	63,348	52,307
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	90,630	111,183
Cost of sales	27,438	38,741
Gross margin	63,192	72,442
Operating, general and administrative	22,301	21,008
Property and other taxes	9,761	9,740
Depreciation and depletion	8,089	7,533
Operating income	23,041	34,161
Interest expense, net	(1,604)	(1,510)
Other income (expense), net	95	(477)
Income tax (expense) benefit	(737)	1,079
Net Income	20,795	33,253
Total assets	1,616,381	1,151,929
Capital expenditures	15,023	13,270
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	(2,184)	2,301
Property and other taxes	2	2
Depreciation and depletion	0	0
Operating income	2,182	(2,303)
Interest expense, net	(1,914)	(2,745)
Other income (expense), net	(2,690)	2,187
Income tax (expense) benefit	1,887	(843)
Net Income	(535)	(3,704)
Total assets	4,619	4,683
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	0	0
Interest expense, net	0	0
Other income (expense), net	0	0
Income tax (expense) benefit	0	0
Net Income	0	0
Total assets	0	0
Capital expenditures	$ 0	$ 0