Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 775,144	$ 822,384
Total revenues from other sources	122,014	105,853
Revenues	897,158	928,237
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,515	12,022
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	762,629	810,362
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	241,522	255,550
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	241,768	254,470
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	271,087	290,978
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,946	5,457
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,515	12,022
Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,306	3,907
Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues from other sources	21,190	15,604
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	87,935	79,479
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	4,694	2,356
Other
Disaggregation of Revenue [Line Items]
Total revenues from other sources	8,195	8,414
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	759,323	806,455
Total revenues from other sources	29,255	23,906
Revenues	788,578	830,361
Electric utility | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Electric utility | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	759,323	806,455
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	241,522	255,550
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	241,768	254,470
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	271,087	290,978
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,946	5,457
Electric utility | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Electric utility | Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues from other sources	21,190	15,604
Electric utility | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0
Electric utility | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0
Electric utility | Other
Disaggregation of Revenue [Line Items]
Total revenues from other sources	8,065	8,302
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,515	12,022
Total revenues from other sources	92,629	81,835
Revenues	105,144	93,857
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,515	12,022
Bank | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Bank | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Bank | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Bank | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Bank | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Bank | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,515	12,022
Bank | Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Bank | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0
Bank | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	87,935	79,479
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	4,694	2,356
Bank | Other
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,306	3,907
Total revenues from other sources	130	112
Revenues	3,436	4,019
Other | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Other | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,306	3,907
Other | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Other | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Other | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Other | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Other | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Other | Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,306	3,907
Other | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0
Other | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0
Other | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	0	0
Other | Other
Disaggregation of Revenue [Line Items]
Total revenues from other sources	$ 130	$ 112