Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Operating revenues	$ 696,475	$ 661,930
Regulatory cost recovery revenue	14,000	14,000
Electric Service | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	340,838	325,073
Electric Service | Non-Residential
Disaggregation of Revenue [Line Items]
Operating revenues	314,783	303,351
Electric Service | Wholesale Energy Sales
Disaggregation of Revenue [Line Items]
Operating revenues	17,597	14,668
Transmission Services for Others
Disaggregation of Revenue [Line Items]
Operating revenues	18,993	15,927
Other Sources
Disaggregation of Revenue [Line Items]
Operating revenues	4,264	2,911
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 682,000	$ 648,000