Revenue - Sources of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total Operating Revenues	$ 3,586,982	$ 3,471,209	$ 3,691,247
Retail Electric Service | Retail residential
Disaggregation of Revenue [Line Items]
Total Operating Revenues	1,929,178	1,761,122	1,867,370
Retail Electric Service | Retail non-residential
Disaggregation of Revenue [Line Items]
Total Operating Revenues	1,486,098	1,509,514	1,628,891
Retail Electric Service | Wholesale
Disaggregation of Revenue [Line Items]
Total Operating Revenues	93,345	121,805	109,198
Transmission Services for Others
Disaggregation of Revenue [Line Items]
Total Operating Revenues	65,859	62,460	60,261
Other Sources
Disaggregation of Revenue [Line Items]
Total Operating Revenues	$ 12,502	$ 16,308	$ 25,527