Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,000,249	$ 929,590	$ 1,696,724	$ 1,591,520
Regulatory cost recovery revenue	20,000	15,000	34,000	29,000
Electric Service | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	531,717	515,128	872,555	840,201
Electric Service | Non-Residential
Disaggregation of Revenue [Line Items]
Operating revenues	420,995	381,121	735,778	684,472
Electric Service | Wholesale Energy Sales
Disaggregation of Revenue [Line Items]
Operating revenues	18,007	15,927	35,604	30,595
Transmission Services for Others
Disaggregation of Revenue [Line Items]
Operating revenues	22,579	14,766	41,572	30,693
Other Sources
Disaggregation of Revenue [Line Items]
Operating revenues	6,951	2,648	11,215	5,559
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 980,000	$ 915,000	$ 1,663,000	$ 1,563,000