Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 577,390	$ 575,465
Total revenues	661,615	645,874
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,233	11,497
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	566,157	563,968
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	175,745	178,589
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	187,408	188,998
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	198,926	192,321
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,078	4,060
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,233	11,497
Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	84,225	70,409
Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	6,207	4,750
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	68,488	62,002
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	3,331	1,920
Other
Disaggregation of Revenue [Line Items]
Total revenues	6,199	1,737
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	566,157	563,968
Total revenues	578,495	570,427
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	566,157	563,968
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	175,745	178,589
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	187,408	188,998
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	198,926	192,321
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,078	4,060
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	12,338	6,459
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	6,207	4,750
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	6,131	1,709
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,233	11,497
Total revenues	83,052	75,419
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,233	11,497
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	11,233	11,497
Bank | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	71,819	63,922
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	68,488	62,002
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	3,331	1,920
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	68	28
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	68	28
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	$ 68	$ 28