Electric plant, construction and related agreements - Narrative (Details) $ in Millions		1 Months Ended	12 Months Ended
	Feb. 18, 2019 USD ($)	Nov. 30, 2021	Dec. 31, 2021 USD ($) MW	Dec. 31, 2008 unit MW
Vogtle Units No. 3 & No. 4
Public Utility Property Plant and Equipment
Ownership interest (as a percent)			30.00%
Total investment in additional Vogtle units			$ 7,000
Monthly delay cost			75
Monthly delay cost, exercise of tender option			30
Additional construction contingency, construction productivity and field support			2,800
Ownership amount of additional construction contingency, construction productivity and field support			825
Additional construction contingency, total			216
Ownership amount of additional project level contingency			$ 65
Remaining share paid by counterparty upon exercise of tender option (as a percent)			100.00%
Budget increases since the nineteenth VCM			$ 3,400
Percentage of disallowed costs excluded from adverse event triggers			6.00%
Vogtle Units No. 3 & No. 4 | Jointly Owned Nuclear Power Plant
Public Utility Property Plant and Equipment
Project budget			$ 8,500
Total project cost			$ 8,100
Release of generating capacity with exercise of tender option (in megawatts) | MW			42
Proportionate ownership share with exercise of tender option (as a percent)			28.00%
Ownership amount of additional project level contingency			$ 98
Vogtle Units No. 3 & No. 4 | Financial Exposure Term One
Public Utility Property Plant and Equipment
Ownership interest (as a percent)			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%
Vogtle Units No. 3 & No. 4 | Minimum
Public Utility Property Plant and Equipment
COVID related costs			$ 350
Ownership amount of COVID related costs			105
Vogtle Units No. 3 & No. 4 | Maximum
Public Utility Property Plant and Equipment
COVID related costs			438
Ownership amount of COVID related costs			$ 131
Vogtle Units No. 3 & No. 4 | Ownership participation agreement
Public Utility Property Plant and Equipment
Ownership interest (as a percent)			30.00%
Ownership share, generating capacity (in megawatts) | MW			660
Project budget			$ 8,500
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
Total project cost			$ 17,100
Additional construction costs	$ 800
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 (as a percent)	55.70%
Additional construction costs, responsibility of co-owner	$ 80
Proportionate share of additional construction costs	$ 44
Proportionate share of construction costs, remaining co-owners (as a percent)	44.30%
Proportionate share of construction costs (as a percent)	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 (as a percent)	65.70%
Additional construction costs, responsibility of co-owner	$ 100
Proportionate share of additional construction costs	$ 55
Proportionate share of construction costs, remaining co-owners (as a percent)	34.30%
Proportionate share of construction costs (as a percent)	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 triggering option to tender ownership	$ 2,100
Vogtle Units No. 3 & No. 4 | Global Amendments to Term Sheet | Minimum
Public Utility Property Plant and Equipment
Ownership approval to change primary construction contractor (as a percent)			90.00%
Exercise period of tender option		120 days
Ownership approval required to continue construction (as a percent)			90.00%
Ownership approval required for material amendments (as a percent)			67.00%
Vogtle Units No. 3 & No. 4 | Global Amendments to Term Sheet | Minimum | Financial Exposure Term One
Public Utility Property Plant and Equipment
Additional construction costs	800
Vogtle Units No. 3 & No. 4 | Global Amendments to Term Sheet | Minimum | Financial Exposure Term Two
Public Utility Property Plant and Equipment
Additional construction costs	1,600
Vogtle Units No. 3 & No. 4 | Global Amendments to Term Sheet | Maximum
Public Utility Property Plant and Equipment
Exercise period of tender option		180 days
Vogtle Units No. 3 & No. 4 | Global Amendments to Term Sheet | Maximum | Financial Exposure Term One
Public Utility Property Plant and Equipment
Additional construction costs	1,600
Vogtle Units No. 3 & No. 4 | Global Amendments to Term Sheet | Maximum | Financial Exposure Term Two
Public Utility Property Plant and Equipment
Additional construction costs	$ 2,100
Vogtle Unit Number 4
Public Utility Property Plant and Equipment
Monthly delay cost			$ 30
Monthly delay cost, exercise of tender option			$ 12