Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 1,188,740	$ 1,212,305
Total Regulatory Liabilities	705,073	792,190
Net Regulatory Assets	483,667	420,115
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	35,030	35,580
Deferral of effects on net margin - Hawk Road Energy Facility | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	16,174	16,636
Deferral of effects on net margin - Hawk Road Energy Facility | Bobby C. Smith Jr. Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	5,610	14,825
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	112,842	74,584
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	3,383	5,557
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	166,491	154,514
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 317,491	357,460
Amortization period, regulatory liability	5 years
Natural gas hedges
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 47,054	131,804
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 998	1,230
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization period, regulatory liability	4 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 26,481	29,494
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	21 years
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 29,562	31,908
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 36,023	29,317
Coal-fired maintenance outage costs | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	60 months
Nuclear refueling outage costs | Minimum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	18 months
Nuclear refueling outage costs | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	24 months
Asset retirement obligations | Ashpond and other
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 358,706	353,212
Asset retirement obligations | Nuclear
Regulatory Assets and Liabilities
Total Regulatory Assets	20,724	32,192
Depreciation expense | Plant Vogtle
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 34,482	35,549
Operating license expected extension period for Plant Vogtle	20 years
Operating license period	40 years
Depreciation expense | Plant Wansley
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 346,710	361,784
Deferred charges related to Vogtle Units No. 3 and No. 4 training costs | Vogtle Units No. 3 & No. 4
Regulatory Assets and Liabilities
Total Regulatory Assets	55,468	54,701
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	139,044	136,827
Deferral of effects on net margin - Smith Energy Facility | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	132,271	136,730
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 9,269	$ 10,591
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	27 years