Vogtle Units No. 3 and No. 4 Construction Project - Narrative (Details) $ in Millions		3 Months Ended				9 Months Ended	12 Months Ended
	Feb. 18, 2019 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) unit	Dec. 31, 2008 unit MW
Vogtle Units No. 3 & No. 4
Public Utility Property Plant and Equipment
Ownership interest (as a percent)		30.00%				30.00%
Total investment in additional Vogtle units		$ 6,800.0				$ 6,800.0
Additional construction contingency, total		300.0	$ 260.0	$ 106.0	$ 375.0
Project-Level Contingency		90.0	78.0	32.0	$ 112.5
Additional construction contingency, construction productivity and field support		278.0	746.0	183.0
Ownership amount of additional construction contingency, construction productivity and field support		$ 83.0	224.0	$ 55.0
Project level contingency			300.0
Ownership amount of project level contingency			$ 90.0
Monthly delay cost						55.0
Total project costs						17,100.0
Total budget revisions						$ 2,400.0
Vogtle Units No. 3 & No. 4 | Financial Exposure Term One
Public Utility Property Plant and Equipment
Ownership interest (as a percent)		30.00%				30.00%
Vogtle Units No. 3 & No. 4 | Financial Exposure Term Two
Public Utility Property Plant and Equipment
Ownership interest (as a percent)		30.00%				30.00%
Vogtle Units No. 3 & No. 4 | Minimum
Public Utility Property Plant and Equipment
COVID related costs						$ 350.0
Ownership amount of COVID related costs						105.0
Vogtle Units No. 3 & No. 4 | Maximum
Public Utility Property Plant and Equipment
COVID related costs						438.0
Ownership amount of COVID related costs						$ 131.0
Vogtle Units No. 3 & No. 4 | Ownership participation agreement
Public Utility Property Plant and Equipment
Number of additional nuclear units | unit						2
Ownership interest (as a percent)		30.00%				30.00%
Project budget						$ 8,250.0
Vogtle Units No. 3 & No. 4 | EPC Agreement | Westinghouse Electric Company LLC and Stone & Webster, Inc.
Public Utility Property Plant and Equipment
Number of nuclear units | unit							2
Generating capacity of each nuclear unit (in megawatts) | MW							1,100
Vogtle Units No. 3 & No. 4 | Services Agreement | Westinghouse Electric Company LLC and Stone & Webster, Inc.
Public Utility Property Plant and Equipment
Written notice period for termination of agreement						30 days
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet
Public Utility Property Plant and Equipment
Project budget	$ 8,400.0
Construction contingency estimate	$ 800.0
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Financial Exposure Term One
Public Utility Property Plant and Equipment
Proportionate share of construction costs, Co-Owner	55.70%
Proportionate share of additional construction costs, Co-Owner	$ 80.0
Proportionate share of additional construction costs	$ 44.0
Proportionate share of construction costs, remaining co-owners	44.30%
Proportionate share of construction costs	24.50%
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Financial Exposure Term Two
Public Utility Property Plant and Equipment
Proportionate share of construction costs, Co-Owner	65.70%
Proportionate share of additional construction costs, Co-Owner	$ 100.0
Proportionate share of additional construction costs	$ 55.0
Proportionate share of construction costs, remaining co-owners	34.30%
Proportionate share of construction costs	19.00%
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Financial Exposure Term Three
Public Utility Property Plant and Equipment
Additional construction costs that triggers option to tender ownership	$ 2,100.0
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Minimum
Public Utility Property Plant and Equipment
Percentage of ownership approval to change primary construction contractor		90.00%				90.00%
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Minimum | Financial Exposure Term One
Public Utility Property Plant and Equipment
Construction contingency estimate	800.0
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Minimum | Financial Exposure Term Two
Public Utility Property Plant and Equipment
Construction contingency estimate	$ 1,600.0
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Minimum | Financial Exposure Term Three
Public Utility Property Plant and Equipment
One time option to tender portion of ownership, exercise period	120 days
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Maximum | Financial Exposure Term One
Public Utility Property Plant and Equipment
Construction contingency estimate	$ 1,600.0
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Maximum | Financial Exposure Term Two
Public Utility Property Plant and Equipment
Construction contingency estimate	$ 2,100.0
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Maximum | Financial Exposure Term Three
Public Utility Property Plant and Equipment
One time option to tender portion of ownership, exercise period	180 days
Vogtle Unit Number 4
Public Utility Property Plant and Equipment
Monthly delay cost						$ 25.0