Summary of significant accounting policies - Asset retirement obligations and other retirement costs (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 25, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Asset retirement obligations
Balance at the beginning of the period		$ 1,135,983	$ 1,070,640
Liabilities settled		(12,404)	(6,988)
Accretion		56,086	54,475	$ 50,473
Deferred accretion		(199)	(853)
Change in cash flow estimates		107,677	18,709
Balance at the end of the period		1,287,143	1,135,983	1,070,640
Coal Combustion Residuals
Increase in the obligation for coal ash decommissioning		102,185	10,775
Fund balances for coal ash pond and landfill decommissioning		140,474	120,536
Forecast
Coal Combustion Residuals
Increase in the obligation for coal ash decommissioning	$ 100,000
Hatch Unit No. 1
Nuclear Decommissioning
Estimated costs based on site study		302,000
Hatch Unit No. 1 | Radiated structures
Nuclear Decommissioning
Estimated costs based on site study		227,000
Hatch Unit No. 1 | Spent fuel management
Nuclear Decommissioning
Estimated costs based on site study		60,000
Hatch Unit No. 1 | Non-radiated structures
Nuclear Decommissioning
Estimated costs based on site study		15,000
Hatch Unit No. 2
Nuclear Decommissioning
Estimated costs based on site study		308,000
Hatch Unit No. 2 | Radiated structures
Nuclear Decommissioning
Estimated costs based on site study		236,000
Hatch Unit No. 2 | Spent fuel management
Nuclear Decommissioning
Estimated costs based on site study		51,000
Hatch Unit No. 2 | Non-radiated structures
Nuclear Decommissioning
Estimated costs based on site study		21,000
Vogtle Unit No. 1
Nuclear Decommissioning
Estimated costs based on site study		282,000
Vogtle Unit No. 1 | Radiated structures
Nuclear Decommissioning
Estimated costs based on site study		200,000
Vogtle Unit No. 1 | Spent fuel management
Nuclear Decommissioning
Estimated costs based on site study		58,000
Vogtle Unit No. 1 | Non-radiated structures
Nuclear Decommissioning
Estimated costs based on site study		24,000
Vogtle Unit No. 2
Nuclear Decommissioning
Estimated costs based on site study		297,000
Vogtle Unit No. 2 | Radiated structures
Nuclear Decommissioning
Estimated costs based on site study		213,000
Vogtle Unit No. 2 | Spent fuel management
Nuclear Decommissioning
Estimated costs based on site study		53,000
Vogtle Unit No. 2 | Non-radiated structures
Nuclear Decommissioning
Estimated costs based on site study		31,000
Nuclear
Asset retirement obligations
Balance at the beginning of the period		738,217	697,441
Liabilities settled		0	0
Accretion		43,206	40,776
Deferred accretion		0	0
Change in cash flow estimates		(3,209)	0
Balance at the end of the period		778,214	738,217	697,441
Coal Ash Pond
Asset retirement obligations
Balance at the beginning of the period		346,589	329,264
Liabilities settled		(17,046)	(4,656)
Accretion		11,157	12,059
Deferred accretion		(199)	(853)
Change in cash flow estimates		102,185	10,775
Balance at the end of the period		442,686	346,589	329,264
Other
Asset retirement obligations
Balance at the beginning of the period		51,177	43,935
Liabilities settled		4,642	(2,332)
Accretion		1,723	1,640
Deferred accretion		0	0
Change in cash flow estimates		8,701	7,934
Balance at the end of the period		$ 66,243	$ 51,177	$ 43,935