Revenue Recognition - Management Program (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2018
Vogtle Units No. 3 & No. 4
Operating revenues
Amounts billed under rate management program	$ 3,857	$ 3,981
Cumulative recovery of financing costs	99,800
Vogtle New Units
Operating revenues
Cumulative recovery of financing costs	254,084
Rate management program, additional collection period			5 years
Rate management program, billing period			5 years
Amounts billed under additional rate management program	$ 40,044	$ 28,488