Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 550,412	$ 725,111	$ 1,701,967	$ 1,962,922
Revenues	641,427	770,882	1,927,558	2,147,982
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,589	12,111	28,356	34,976
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	540,823	713,000	1,673,611	1,927,946
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	191,321	230,051	569,177	601,664
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	171,156	230,411	528,135	635,097
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	176,200	248,457	568,887	679,252
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,935	4,081	7,172	11,933
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,589	12,111	28,356	34,976
Solar energy sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	211		240
Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Revenues	91,015	45,771	225,591	185,060
Regulatory revenue
Disaggregation of Revenue [Line Items]
Revenues	15,457	(30,800)	2,979	(44,953)
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Revenues	59,640	66,859	184,444	201,502
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Revenues	9,415	3,578	20,296	10,809
Other
Disaggregation of Revenue [Line Items]
Revenues	6,503	6,134	17,872	17,702
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	540,612	713,000	1,673,371	1,927,946
Revenues	562,568	688,330	1,694,225	1,900,609
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	540,612	713,000	1,673,371	1,927,946
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	191,321	230,051	569,177	601,664
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	171,156	230,411	528,135	635,097
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	176,200	248,457	568,887	679,252
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,935	4,081	7,172	11,933
Electric utility | Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Electric utility | Solar energy sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Electric utility | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Revenues	21,956	(24,670)	20,854	(27,337)
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Revenues	15,457	(30,800)	2,979	(44,953)
Electric utility | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Electric utility | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Electric utility | Other
Disaggregation of Revenue [Line Items]
Revenues	6,499	6,130	17,875	17,616
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,589	12,111	28,356	34,976
Revenues	78,644	82,548	233,096	247,287
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,589	12,111	28,356	34,976
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	9,589	12,111	28,356	34,976
Bank | Solar energy sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0		0
Bank | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Revenues	69,055	70,437	204,740	212,311
Bank | Regulatory revenue
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Bank | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Revenues	59,640	66,859	184,444	201,502
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Revenues	9,415	3,578	20,296	10,809
Bank | Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	211	0	240	0
Revenues	215	4	237	86
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	211	0	240	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 | Solar energy sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	211		240
Other | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Revenues	4	4	(3)	86
Other | Regulatory revenue
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Other | Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Other | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Other | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 4	$ 4	$ (3)	$ 86