Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 725,111	$ 710,168	$ 1,962,922	$ 1,907,220
Total revenues	771,535	768,048	2,148,635	2,099,199
Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,111	12,575	34,976	37,177
Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	713,000	697,593	1,927,946	1,870,043
Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	230,051	222,196	601,664	586,002
Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	230,411	229,476	635,097	624,643
Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	248,457	242,457	679,252	649,454
Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,081	4,296	11,933	12,324
Bank fees
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,111	11,743	34,976	34,797
Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	46,424	57,880	185,713	191,979
Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	(30,800)	(13,572)	(44,953)	(13,465)
Bank interest and dividend income
Disaggregation of Revenue [Line Items]
Total revenues	66,859	65,185	201,502	190,448
Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	4,231	3,568	11,462	7,774
Other
Disaggregation of Revenue [Line Items]
Total revenues	6,134	2,699	17,702	7,222
Electric utility
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	713,000	698,425	1,927,946	1,872,423
Total revenues	688,330	687,409	1,900,609	1,865,962
Electric utility | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	832	0	2,380
Electric utility | Services/goods transferred over time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	713,000	697,593	1,927,946	1,870,043
Electric utility | Electric energy sales - residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	230,051	222,196	601,664	586,002
Electric utility | Electric energy sales - commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	230,411	229,476	635,097	624,643
Electric utility | Electric energy sales - large light and power
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	248,457	242,457	679,252	649,454
Electric utility | Electric energy sales - other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,081	4,296	11,933	12,324
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	(24,670)	(11,016)	(27,337)	(6,461)
Electric utility | Regulatory revenue
Disaggregation of Revenue [Line Items]
Total revenues	(30,800)	(13,572)	(44,953)	(13,465)
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	6,130	2,556	17,616	7,004
Bank
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,111	11,743	34,976	34,797
Total revenues	83,201	80,496	247,940	233,019
Bank | Services/goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,111	11,743	34,976	34,797
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	12,111	11,743	34,976	34,797
Bank | Revenues from Other Sources
Disaggregation of Revenue [Line Items]
Total revenues	71,090	68,753	212,964	198,222
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	66,859	65,185	201,502	190,448
Bank | Other bank noninterest income
Disaggregation of Revenue [Line Items]
Total revenues	4,231	3,568	11,462	7,774
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	4	143	86	218
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	4	143	86	218
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	$ 4	$ 143	$ 86	$ 218