Investments - Investment in associated companies (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investment in associated companies
Investment in associated companies	$ 73,318	$ 77,037
National Rural Utilities Cooperative Finance Corporation (CFC)
Investment in associated companies
Investment in associated companies	24,065	24,061
CT Parts, LLC
Investment in associated companies
Investment in associated companies	7,175	10,236
Georgia Transmission Corporation
Investment in associated companies
Investment in associated companies	32,106	30,237
Georgia System Operations Corporation
Investment in associated companies
Investment in associated companies	7,000	9,250
Other
Investment in associated companies
Investment in associated companies	$ 2,972	$ 3,253