Revenue Recognition - Management Program (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2018
Vogtle Units No. 3 & No. 4
Operating revenues
Amounts billed under rate management program	$ 11,601	$ 11,805
Cumulative recovery of financing costs	107,544
Vogtle New Units
Operating revenues
Cumulative recovery of financing costs	330,165
Rate management program, additional collection period			5 years
Rate management program, billing period			5 years
Amounts billed under additional rate management program	115,837	$ 93,390
Effingham County Power
Operating revenues
Cumulative recovery of financing costs	$ 191