Summary of significant accounting policies - Revenue recognition (Details) $ in Thousands	12 Months Ended			60 Months Ended
	Dec. 31, 2022 USD ($) member service	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2022 USD ($)
Revenue Recognition
Number of electric distribution cooperative members | member	38
Number of services provided | service	2
Energy supplied to members, percent	58.00%	62.00%	57.00%
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	$ 28,471	$ 30,029		$ 28,471
Total revenues	2,130,137	1,604,863	$ 1,377,618
Vogtle Units No. 3 & No. 4
Operating revenues
Recovery of financing costs	14,796	15,693	14,684
Cumulative recovery of financing costs	$ 126,432
Additional collection period (in years)	5 years
Billed amount	$ 11,774	143,000	125,842	369,102
Members
Revenue Recognition
Total revenues	1,974,683	1,557,109	1,377,010
Operating revenues
Receivables	$ 187,401	$ 143,715	$ 135,462	187,401
Jackson EMC | Total operating revenues | Revenues of members
Operating revenues
Concentration risk (as a percent)	16.00%	15.20%	15.20%
GreyStone Power Corporation, an EMC | Total operating revenues | Revenues of members
Operating revenues
Concentration risk (as a percent)	10.00%	8.70%	8.70%
Cobb EMC | Total operating revenues | Revenues of members
Operating revenues
Concentration risk (as a percent)	9.50%	12.30%	13.20%
Non-Members
Revenue Recognition
Total revenues	$ 155,454	$ 47,754	$ 608
Operating revenues
Receivables	32,614			$ 32,614
Capacity revenues | Members
Revenue Recognition
Total revenues	984,036	946,662	971,071
Capacity revenues | Non-Members
Revenue Recognition
Total revenues	82	0	0
Energy revenues | Members
Revenue Recognition
Total revenues	990,647	610,447	405,939
Energy revenues | Non-Members
Revenue Recognition
Total revenues	$ 155,372	$ 47,754	$ 608