Summary of significant accounting policies - Asset retirement obligations and other retirement costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Asset retirement obligations
Balance at the beginning of the period	$ 1,017,563	$ 734,997
Liabilities settled	(4,538)	(4,680)
Accretion	50,473	38,090	$ 36,674
Deferred accretion	1,860
Change in cash flow estimates	5,282	249,156
Balance at the end of the period	1,070,640	1,017,563	734,997
Coal Combustion Residuals
Increase in the obligation for coal ash decommissioning	5,958	161,303
Fund balances for coal ash pond and landfill decommissioning	93,184	$ 60,599
Hatch Units No. 1 & No. 2
Nuclear Decommissioning
Assumed escalation rate for labor, material and equipment (as a percent)		2.80%
Hatch Unit No. 1
Nuclear Decommissioning
Estimated costs based on site study	277,000
Hatch Unit No. 1 | Radiated structures
Nuclear Decommissioning
Estimated costs based on site study	209,000
Hatch Unit No. 1 | Spent fuel management
Nuclear Decommissioning
Estimated costs based on site study	54,000
Hatch Unit No. 1 | Non-radiated structures
Nuclear Decommissioning
Estimated costs based on site study	14,000
Hatch Unit No. 2
Nuclear Decommissioning
Estimated costs based on site study	299,000
Hatch Unit No. 2 | Radiated structures
Nuclear Decommissioning
Estimated costs based on site study	231,000
Hatch Unit No. 2 | Spent fuel management
Nuclear Decommissioning
Estimated costs based on site study	49,000
Hatch Unit No. 2 | Non-radiated structures
Nuclear Decommissioning
Estimated costs based on site study	19,000
Vogtle Units No. 1 & No. 2
Nuclear Decommissioning
Assumed escalation rate for labor, material and equipment (as a percent)		2.70%
Vogtle Unit No. 1
Nuclear Decommissioning
Estimated costs based on site study	266,000
Vogtle Unit No. 1 | Radiated structures
Nuclear Decommissioning
Estimated costs based on site study	188,000
Vogtle Unit No. 1 | Spent fuel management
Nuclear Decommissioning
Estimated costs based on site study	55,000
Vogtle Unit No. 1 | Non-radiated structures
Nuclear Decommissioning
Estimated costs based on site study	23,000
Vogtle Unit No. 2
Nuclear Decommissioning
Estimated costs based on site study	286,000
Vogtle Unit No. 2 | Radiated structures
Nuclear Decommissioning
Estimated costs based on site study	206,000
Vogtle Unit No. 2 | Spent fuel management
Nuclear Decommissioning
Estimated costs based on site study	51,000
Vogtle Unit No. 2 | Non-radiated structures
Nuclear Decommissioning
Estimated costs based on site study	29,000
Nuclear
Asset retirement obligations
Balance at the beginning of the period	658,956	$ 548,574
Liabilities settled	0	(1,686)
Accretion	38,485	32,857
Deferred accretion	0
Change in cash flow estimates	0	79,211
Balance at the end of the period	697,441	658,956	548,574
Coal Ash Pond
Asset retirement obligations
Balance at the beginning of the period	326,248	161,755
Liabilities settled	(3,380)	(1,596)
Accretion	10,494	4,238
Deferred accretion	1,860
Change in cash flow estimates	(5,958)	161,851
Balance at the end of the period	329,264	326,248	161,755
Other
Asset retirement obligations
Balance at the beginning of the period	32,359	24,668
Liabilities settled	(1,158)	(1,398)
Accretion	1,494	995
Deferred accretion	0
Change in cash flow estimates	11,240	8,094
Balance at the end of the period	$ 43,935	$ 32,359	$ 24,668