Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 618,605	$ 577,390
Total revenues	677,186	661,615
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,556	11,233
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	607,049	566,157
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	190,266	175,745
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	197,105	187,408
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	216,220	198,926
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,458	4,078
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,556	11,233
Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	58,581	84,225
Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	(15,304)	6,207
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	64,975	68,488
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	3,207	3,331
Other
Disaggregation of Revenue [Line Items]
Total revenues	5,703	6,199
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	607,049	566,157
Total revenues	597,442	578,495
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	607,049	566,157
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	190,266	175,745
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	197,105	187,408
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	216,220	198,926
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,458	4,078
Electric utility | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Electric utility | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	(9,607)	12,338
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	(15,304)	6,207
Electric utility | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Electric utility | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Electric utility | Other
Disaggregation of Revenue [Line Items]
Total revenues	5,697	6,131
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,556	11,233
Total revenues	79,738	83,052
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,556	11,233
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	11,556	11,233
Bank | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	68,182	71,819
Bank | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Bank | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	64,975	68,488
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	3,207	3,331
Bank | Other
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Total revenues	6	68
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	0	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 | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	6	68
Other | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Other | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Other | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	0	0
Other | Other
Disaggregation of Revenue [Line Items]
Total revenues	$ 6	$ 68