Revenue - Schedule of Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,308,254	$ 1,254,501	$ 3,004,978	$ 2,846,021
Regulatory cost recovery revenue	4,000	11,000	38,000	40,000
Electric Service | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	681,918	726,231	1,554,473	1,566,432
Electric Service | Non-Residential
Disaggregation of Revenue [Line Items]
Operating revenues	480,671	461,168	1,216,449	1,145,640
Electric Service | Wholesale Energy Sales
Disaggregation of Revenue [Line Items]
Operating revenues	108,539	45,631	144,143	76,226
Transmission Services for Others
Disaggregation of Revenue [Line Items]
Operating revenues	35,816	18,000	77,388	48,693
Other Sources
Disaggregation of Revenue [Line Items]
Operating revenues	1,310	3,471	12,525	9,030
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,304,000	$ 1,244,000	$ 2,967,000	$ 2,806,000