Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 822,044	$ 990,338	$ 2,435,416	$ 2,527,153
Total revenues from other sources	79,829	51,859	290,379	195,719
Revenues	901,873	1,042,197	2,725,795	2,722,872
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,067	12,058	36,370	36,851
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	809,977	978,280	2,399,046	2,490,302
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	257,790	306,417	753,207	790,424
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	257,876	309,940	762,454	794,238
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	284,607	351,763	855,396	886,733
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,688	5,400	14,628	11,699
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,067	12,058	36,370	36,851
Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,016	4,760	13,361	7,208
Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues from other sources	(20,927)	(26,301)	3,716	(24,843)
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	85,636	68,417	246,856	191,228
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	3,271	936	8,490	3,771
Other
Disaggregation of Revenue [Line Items]
Total revenues from other sources	11,849	8,807	31,317	25,563
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	804,961	973,520	2,385,685	2,483,094
Total revenues from other sources	(9,974)	(17,549)	33,854	542
Revenues	794,987	955,971	2,419,539	2,483,636
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	804,961	973,520	2,385,685	2,483,094
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	257,790	306,417	753,207	790,424
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	257,876	309,940	762,454	794,238
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	284,607	351,763	855,396	886,733
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,688	5,400	14,628	11,699
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	(20,927)	(26,301)	3,716	(24,843)
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	10,953	8,752	30,138	25,385
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,067	12,058	36,370	36,851
Total revenues from other sources	88,907	69,353	255,346	194,999
Revenues	100,974	81,411	291,716	231,850
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,067	12,058	36,370	36,851
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	12,067	12,058	36,370	36,851
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	85,636	68,417	246,856	191,228
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	3,271	936	8,490	3,771
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	5,016	4,760	13,361	7,208
Total revenues from other sources	896	55	1,179	178
Revenues	5,912	4,815	14,540	7,386
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	5,016	4,760	13,361	7,208
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	5,016	4,760	13,361	7,208
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	$ 896	$ 55	$ 1,179	$ 178