Segment and Related Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 328,135	$ 274,836	$ 270,719	$ 384,220	$ 308,816	$ 279,874	$ 261,817	$ 341,502	$ 1,257,910	$ 1,192,009	$ 1,305,652
Cost of sales									318,020	272,883	410,349
Gross margin									939,890	919,126	895,303
Operating, general and administrative									318,229	307,119	294,803
Property and other taxes									171,888	171,259	162,614
Depreciation and depletion									172,923	174,476	166,137
Operating Income	84,651	46,356	48,823	97,020	64,742	47,808	69,210	84,512	276,850	266,272	271,749
Interest expense, net									(95,068)	(91,988)	(92,263)
Other income, net									413	3,966	(3,415)
Income tax (expense) benefit									19,925	18,710	(13,368)
Net Income	59,982	$ 21,670	$ 47,662	$ 72,806	66,492	$ 28,182	$ 43,787	$ 58,499	202,120	196,960	162,703
Total assets	5,910,702				5,644,376				5,910,702	5,644,376	5,420,917
Capital expenditures									316,016	283,966	276,438
Electric
Segment Reporting Information [Line Items]
Total revenues									981,178	921,093	1,037,053
Cost of sales									239,589	194,608	334,029
Gross margin									741,589	726,485	703,024
Operating, general and administrative									232,424	223,598	216,003
Property and other taxes									134,686	134,681	127,391
Depreciation and depletion									143,262	144,636	136,556
Operating Income									231,217	223,570	223,074
Interest expense, net									(78,809)	(79,033)	(82,454)
Other income, net									(1,365)	2,794	(3,487)
Income tax (expense) benefit									(6,079)	21,686	(7,424)
Net Income									144,964	169,017	129,709
Total assets	4,685,990				4,512,392				4,685,990	4,512,392	4,346,484
Capital expenditures									241,190	221,968	226,077
Gas
Segment Reporting Information [Line Items]
Total revenues									276,732	270,916	268,599
Cost of sales									78,431	78,275	76,320
Gross margin									198,301	192,641	192,279
Operating, general and administrative									82,732	82,864	78,757
Property and other taxes									37,192	36,569	35,214
Depreciation and depletion									29,661	29,822	29,548
Operating Income									48,716	43,386	48,760
Interest expense, net									(6,218)	(5,858)	(5,920)
Other income, net									(814)	962	(878)
Income tax (expense) benefit									493	9,268	(6,684)
Net Income									42,177	47,758	35,278
Total assets	1,220,048				1,127,252				1,220,048	1,127,252	1,071,847
Capital expenditures									74,826	61,998	50,361
Other Segments [Member]
Segment Reporting Information [Line Items]
Total revenues									0	0	0
Cost of sales									0	0	0
Gross margin									0	0	0
Operating, general and administrative									3,073	657	43
Property and other taxes									10	9	9
Depreciation and depletion									0	18	33
Operating Income									(3,083)	(684)	(85)
Interest expense, net									(10,041)	(7,097)	(3,889)
Other income, net									2,592	210	950
Income tax (expense) benefit									25,511	(12,244)	740
Net Income									14,979	(19,815)	(2,284)
Total assets	4,664				4,732				4,664	4,732	2,586
Capital expenditures									0	0	0
Eliminations
Segment Reporting Information [Line Items]
Total revenues									0	0	0
Cost of sales									0	0	0
Gross margin									0	0	0
Operating, general and administrative									0	0	0
Property and other taxes									0	0	0
Depreciation and depletion									0	0	0
Operating Income									0	0	0
Interest expense, net									0	0	0
Other income, net									0	0	0
Income tax (expense) benefit									0	0	0
Net Income									0	0	0
Total assets	$ 0				$ 0				0	0	0
Capital expenditures									$ 0	$ 0	$ 0