Revenues (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
General payment period		30 days
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 710,168	$ 1,907,220
Total revenues	768,048	2,099,199
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,575	37,177
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	697,593	1,870,043
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	222,196	586,002
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	229,476	624,643
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	242,457	649,454
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,464	9,944
Utility fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	832	2,380
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,743	34,797
Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	57,880	191,979
Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	(13,572)	(13,465)
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	65,185	190,448
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	3,568	7,774
Other
Disaggregation of Revenue [Line Items]
Total revenues	2,699	7,222
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	698,425	1,872,423
Total revenues	687,409	1,865,962
Electric utility | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	832	2,380
Electric utility | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	697,593	1,870,043
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	222,196	586,002
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	229,476	624,643
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	242,457	649,454
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,464	9,944
Electric utility | Utility fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	832	2,380
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	(11,016)	(6,461)
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	(13,572)	(13,465)
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	2,556	7,004
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,743	34,797
Total revenues	80,496	233,019
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,743	34,797
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 | Utility fees
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	11,743	34,797
Bank | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	68,753	198,222
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	65,185	190,448
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	3,568	7,774
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	143	218
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 | Utility fees
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	143	218
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	$ 143	$ 218