Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 660,421	$ 621,587	$ 1,237,811	$ 1,197,052
Total revenues	715,485	685,277	1,377,100	1,331,151
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,632	12,308	22,865	24,602
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	648,789	609,279	1,214,946	1,172,450
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	195,868	185,217	371,613	363,806
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	217,278	206,169	404,686	395,167
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	231,869	214,676	430,795	406,997
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,774	3,968	7,852	8,028
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,632	11,557	22,865	23,054
Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	55,064	63,690	139,289	134,099
Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	(20,360)	(4,643)	(14,153)	107
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	66,155	63,261	134,643	125,263
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	3,900	2,286	7,231	4,206
Other
Disaggregation of Revenue [Line Items]
Total revenues	5,369	2,786	11,568	4,523
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	648,789	610,030	1,214,946	1,173,998
Total revenues	633,784	608,126	1,212,279	1,178,553
Electric utility | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	751	0	1,548
Electric utility | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	648,789	609,279	1,214,946	1,172,450
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	195,868	185,217	371,613	363,806
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	217,278	206,169	404,686	395,167
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	231,869	214,676	430,795	406,997
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,774	3,968	7,852	8,028
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	(15,005)	(1,904)	(2,667)	4,555
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	(20,360)	(4,643)	(14,153)	107
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	5,355	2,739	11,486	4,448
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,632	11,557	22,865	23,054
Total revenues	81,687	77,104	164,739	152,523
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,632	11,557	22,865	23,054
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	11,632	11,557	22,865	23,054
Bank | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	70,055	65,547	141,874	129,469
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	66,155	63,261	134,643	125,263
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	3,900	2,286	7,231	4,206
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	14	47	82	75
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	14	47	82	75
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	$ 14	$ 47	$ 82	$ 75