Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue from Contract with Customer [Abstract]
General payment period	30 days
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 575,465
Other revenues	70,409
Total revenues	645,874	$ 591,562
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,294
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	563,171
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	178,589
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	188,998
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	192,321
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,263
Utility fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	797
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,497
Regulatory revenue
Disaggregation of Revenue [Line Items]
Other revenues	4,750
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Other revenues	62,002
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Other revenues	1,920
Other
Disaggregation of Revenue [Line Items]
Other revenues	1,737
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	563,968
Other revenues	6,459
Total revenues	570,427	518,611
Electric utility | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	797
Electric utility | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	563,171
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	178,589
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	188,998
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	192,321
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,263
Electric utility | Utility fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	797
Electric utility | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Other revenues	4,750
Electric utility | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Other revenues	0
Electric utility | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Other revenues	0
Electric utility | Other
Disaggregation of Revenue [Line Items]
Other revenues	1,709
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,497
Other revenues	63,922
Total revenues	75,419	72,856
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,497
Bank | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Bank | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Bank | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Bank | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Bank | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Bank | Utility fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Bank | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,497
Bank | Regulatory revenue
Disaggregation of Revenue [Line Items]
Other revenues	0
Bank | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Other revenues	62,002
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Other revenues	1,920
Bank | Other
Disaggregation of Revenue [Line Items]
Other revenues	0
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Other revenues	28
Total revenues	28	$ 95
Other | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Other | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Other | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Other | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Other | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Other | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Other | Utility fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Other | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0
Other | Regulatory revenue
Disaggregation of Revenue [Line Items]
Other revenues	0
Other | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Other revenues	0
Other | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Other revenues	0
Other | Other
Disaggregation of Revenue [Line Items]
Other revenues	$ 28