Investments - Investment in associated companies (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Investment in associated companies
Investment in associated companies	$ 75,826	$ 74,844
National Rural Utilities Cooperative Finance Corporation (CFC)
Investment in associated companies
Investment in associated companies	24,081	24,080
CT Parts, LLC
Investment in associated companies
Investment in associated companies	7,049	6,554
Georgia Transmission Corporation
Investment in associated companies
Investment in associated companies	35,696	33,943
Georgia System Operations Corporation
Investment in associated companies
Investment in associated companies	6,500	7,500
Other
Investment in associated companies
Investment in associated companies	$ 2,500	$ 2,767