Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,254,501	$ 1,190,787	$ 2,846,021	$ 2,800,818
Regulatory cost recovery revenue	11,000	13,000	40,000	45,000
Electric Service | Residential
Disaggregation of Revenue [Line Items]
Operating revenues	726,231	668,467	1,566,432	1,452,601
Electric Service | Non-Residential
Disaggregation of Revenue [Line Items]
Operating revenues	461,168	465,602	1,145,640	1,194,199
Electric Service | Wholesale energy sales
Disaggregation of Revenue [Line Items]
Operating revenues	45,631	36,775	76,226	95,218
Transmission services for others
Disaggregation of Revenue [Line Items]
Operating revenues	18,000	15,841	48,693	46,247
Other sources
Disaggregation of Revenue [Line Items]
Operating revenues	3,471	4,102	9,030	12,553
Electric and Transmission Service
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,244,000	$ 1,178,000	$ 2,806,000	$ 2,756,000