Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 541,300	$ 618,605
Revenues	642,946	677,186
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,378	11,556
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	529,922	607,049
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	181,239	190,266
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	168,465	197,105
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	176,815	216,220
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,479	3,458
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,378	11,556
Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	924
Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Revenues	101,646	58,581
Regulatory revenue
Disaggregation of Revenue [Line Items]
Revenues	28,429	(15,304)
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Revenues	58,620	64,975
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Revenues	7,133	3,207
Other
Disaggregation of Revenue [Line Items]
Revenues	7,464	5,703
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	528,998	607,049
Revenues	564,864	597,442
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	528,998	607,049
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	181,239	190,266
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	168,465	197,105
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	176,815	216,220
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,479	3,458
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
Electric utility | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Revenues	35,866	(9,607)
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Revenues	28,429	(15,304)
Electric utility | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Revenues	0	0
Electric utility | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Revenues	0	0
Electric utility | Other
Disaggregation of Revenue [Line Items]
Revenues	7,437	5,697
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,378	11,556
Revenues	77,131	79,738
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,378	11,556
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	11,378	11,556
Bank | Other sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Bank | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Revenues	65,753	68,182
Bank | Regulatory revenue
Disaggregation of Revenue [Line Items]
Revenues	0	0
Bank | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Revenues	58,620	64,975
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Revenues	7,133	3,207
Bank | Other
Disaggregation of Revenue [Line Items]
Revenues	0	0
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	924	0
Revenues	951	6
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	924	0
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	924
Other | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Revenues	27	6
Other | Regulatory revenue
Disaggregation of Revenue [Line Items]
Revenues	0	0
Other | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Revenues	0	0
Other | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Revenues	0	0
Other | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 27	$ 6