Summary of significant accounting policies - Revenue recognition (Details) $ in Thousands	12 Months Ended			48 Months Ended
	Dec. 31, 2021 USD ($) service member	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2021 USD ($)
Revenue Recognition
Number of electric distribution cooperative members | member	38
Number of services provided | service	2
Energy supplied to members, percent	62.00%	57.00%	58.00%
Minimum margins for interest ratio under the first mortgage indenture	1.10	1.10	1.10
Targeted margins for interest ratio	1.14	1.14	1.14
Refund liability	$ 30,029	$ 32,318		$ 30,029
Total revenues	1,604,863	1,377,618	$ 1,430,292
Vogtle Units No. 3 & No. 4
Operating revenues
Recovery of financing costs	15,693	14,684	14,943
Cumulative recovery of financing costs	$ 111,636
Additional collection period (in years)	5 years
Billed amount	$ 143,000	125,842	73,051	357,328
Members
Revenue Recognition
Total revenues	1,557,109	1,377,010	1,429,852
Operating revenues
Receivables	$ 143,715	$ 135,462	$ 142,946	$ 143,715
Jackson EMC | Total operating revenues | Revenues of members
Operating revenues
Concentration risk (as a percent)	15.20%	15.20%	14.40%
Cobb EMC | Total operating revenues | Revenues of members
Operating revenues
Concentration risk (as a percent)	12.30%	13.20%	13.80%
Non-Members
Revenue Recognition
Total revenues	$ 47,754	$ 608	$ 440
Capacity revenues | Members
Revenue Recognition
Total revenues	946,662	971,071	942,057
Energy revenues | Members
Revenue Recognition
Total revenues	610,447	405,939	487,795
Energy revenues | Non-Members
Revenue Recognition
Total revenues	$ 47,754	$ 608	$ 440