Summary of significant accounting policies - Asset retirement obligations and other retirement costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Asset retirement obligations
Balance at the beginning of the period	$ 1,287,143	$ 1,135,983
Liabilities settled	(10,318)	(12,404)
Accretion	55,953	56,086	$ 54,475
Deferred accretion	479	(199)
Change in cash flow estimates	10,486	107,677
Balance at the end of the period	1,343,743	1,287,143	1,135,983
Coal Combustion Residuals
Increase in the obligation for coal ash decommissioning	16,301	102,185
Fund balances for coal ash pond and landfill decommissioning	153,208	140,474
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	778,214	738,217
Liabilities settled	0	0
Accretion	41,892	43,206
Deferred accretion	0	0
Change in cash flow estimates	0	(3,209)
Balance at the end of the period	820,106	778,214	738,217
Coal Ash Pond
Asset retirement obligations
Balance at the beginning of the period	442,686	346,589
Liabilities settled	(10,134)	(17,046)
Accretion	12,196	11,157
Deferred accretion	479	(199)
Change in cash flow estimates	16,301	102,185
Balance at the end of the period	461,528	442,686	346,589
Other
Asset retirement obligations
Balance at the beginning of the period	66,243	51,177
Liabilities settled	(184)	4,642
Accretion	1,865	1,723
Deferred accretion	0	0
Change in cash flow estimates	(5,815)	8,701
Balance at the end of the period	$ 62,109	$ 66,243	$ 51,177