Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 856,404	$ 822,044	$ 2,428,805	$ 2,435,416
Total revenues from other sources	81,979	79,829	304,096	290,379
Revenues	938,383	901,873	2,732,901	2,725,795
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	13,475	12,067	38,713	36,370
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	842,929	809,977	2,390,092	2,399,046
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	272,541	257,790	758,602	753,207
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	268,538	257,876	761,788	762,454
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	294,421	284,607	846,448	855,396
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,532	4,688	14,087	14,628
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	13,475	12,067	38,713	36,370
Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,897	5,016	9,167	13,361
Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues from other sources	(19,924)	(20,927)	780	3,716
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	87,655	85,636	261,768	246,856
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	4,014	3,271	11,750	8,490
Other
Disaggregation of Revenue [Line Items]
Total revenues from other sources	10,234	11,849	29,798	31,317
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	840,032	804,961	2,380,925	2,385,685
Total revenues from other sources	(10,415)	(9,974)	29,601	33,854
Revenues	829,617	794,987	2,410,526	2,419,539
Electric utility | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Electric utility | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	840,032	804,961	2,380,925	2,385,685
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	272,541	257,790	758,602	753,207
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	268,538	257,876	761,788	762,454
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	294,421	284,607	846,448	855,396
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,532	4,688	14,087	14,628
Electric utility | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Electric utility | Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues from other sources	(19,924)	(20,927)	780	3,716
Electric utility | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0	0	0
Electric utility | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0	0	0
Electric utility | Other
Disaggregation of Revenue [Line Items]
Total revenues from other sources	9,509	10,953	28,821	30,138
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	13,475	12,067	38,713	36,370
Total revenues from other sources	91,669	88,907	273,518	255,346
Revenues	105,144	100,974	312,231	291,716
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	13,475	12,067	38,713	36,370
Bank | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	13,475	12,067	38,713	36,370
Bank | Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0	0	0
Bank | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	87,655	85,636	261,768	246,856
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	4,014	3,271	11,750	8,490
Bank | Other
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0	0	0
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,897	5,016	9,167	13,361
Total revenues from other sources	725	896	977	1,179
Revenues	3,622	5,912	10,144	14,540
Other | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,897	5,016	9,167	13,361
Other | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,897	5,016	9,167	13,361
Other | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0	0	0
Other | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0	0	0
Other | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0	0	0
Other | Other
Disaggregation of Revenue [Line Items]
Total revenues from other sources	$ 725	$ 896	$ 977	$ 1,179