Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 790,988	$ 835,984	$ 1,613,372	$ 1,536,815
Total revenues from other sources	104,697	59,623	210,550	143,860
Revenues	895,685	895,607	1,823,922	1,680,675
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,281	11,797	24,303	24,793
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	778,707	824,187	1,589,069	1,512,022
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	239,867	259,433	495,417	484,007
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	250,108	264,701	504,578	484,298
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	279,811	293,847	570,789	534,970
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,483	4,873	9,940	6,299
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,281	11,797	24,303	24,793
Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,438	1,333	8,345	2,448
Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues from other sources	9,039	(11,428)	24,643	1,458
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	81,741	62,822	161,220	122,811
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	2,863	705	5,219	2,835
Other
Disaggregation of Revenue [Line Items]
Total revenues from other sources	11,054	7,524	19,468	16,756
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	774,269	822,854	1,580,724	1,509,574
Total revenues from other sources	19,922	(3,981)	43,828	18,091
Revenues	794,191	818,873	1,624,552	1,527,665
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	774,269	822,854	1,580,724	1,509,574
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	239,867	259,433	495,417	484,007
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	250,108	264,701	504,578	484,298
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	279,811	293,847	570,789	534,970
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,483	4,873	9,940	6,299
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	9,039	(11,428)	24,643	1,458
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	10,883	7,447	19,185	16,633
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,281	11,797	24,303	24,793
Total revenues from other sources	84,604	63,527	166,439	125,646
Revenues	96,885	75,324	190,742	150,439
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,281	11,797	24,303	24,793
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	12,281	11,797	24,303	24,793
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	81,741	62,822	161,220	122,811
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues from other sources	2,863	705	5,219	2,835
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	4,438	1,333	8,345	2,448
Total revenues from other sources	171	77	283	123
Revenues	4,609	1,410	8,628	2,571
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	4,438	1,333	8,345	2,448
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	4,438	1,333	8,345	2,448
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	$ 171	$ 77	$ 283	$ 123