Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 532,921	$ 660,421	$ 1,151,526	$ 1,237,811
Total revenues	608,945	715,485	1,286,131	1,377,100
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,211	11,632	18,767	22,865
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	525,710	648,789	1,132,759	1,214,946
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	187,590	195,868	377,856	371,613
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	159,874	217,278	356,979	404,686
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	176,467	231,869	392,687	430,795
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,779	3,774	5,237	7,852
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,211	11,632	18,767	22,865
Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	76,024	55,064	134,605	139,289
Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	2,826	(20,360)	(12,478)	(14,153)
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	59,829	66,155	124,804	134,643
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	7,674	3,900	10,881	7,231
Other
Disaggregation of Revenue [Line Items]
Total revenues	5,695	5,369	11,398	11,568
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	525,710	648,789	1,132,759	1,214,946
Total revenues	534,215	633,784	1,131,657	1,212,279
Electric utility | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Electric utility | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	525,710	648,789	1,132,759	1,214,946
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	187,590	195,868	377,856	371,613
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	159,874	217,278	356,979	404,686
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	176,467	231,869	392,687	430,795
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,779	3,774	5,237	7,852
Electric utility | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Electric utility | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	8,505	(15,005)	(1,102)	(2,667)
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	2,826	(20,360)	(12,478)	(14,153)
Electric utility | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Electric utility | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Electric utility | Other
Disaggregation of Revenue [Line Items]
Total revenues	5,679	5,355	11,376	11,486
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,211	11,632	18,767	22,865
Total revenues	74,714	81,687	154,452	164,739
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,211	11,632	18,767	22,865
Bank | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Bank | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,211	11,632	18,767	22,865
Bank | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	67,503	70,055	135,685	141,874
Bank | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Bank | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	59,829	66,155	124,804	134,643
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	7,674	3,900	10,881	7,231
Bank | Other
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Total revenues	16	14	22	82
Other | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	16	14	22	82
Other | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Other | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Other | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
Other | Other
Disaggregation of Revenue [Line Items]
Total revenues	$ 16	$ 14	$ 22	$ 82