Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 1,136,248	$ 1,131,489
Total Regulatory Liabilities	704,084	706,320
Net Regulatory Assets	432,164	425,169
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	23,455	25,992
Deferral of effects on net margin - Hawk Road Energy Facility | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	15,866	16,020
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	114,484	120,547
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	174,507	170,466
Asset retirement obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	82,606	47,217
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 279,467	308,507
Amortization period, regulatory liability	5 years
Natural gas hedges
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 10,381	13,445
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 3,318	4,126
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization period, regulatory liability	24 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 24,504	25,476
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	20 years
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 27,760	28,780
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 34,627	30,040
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	$ 338,752	343,523
Depreciation expense | Plant Vogtle
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 33,770	34,125
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	$ 330,945	335,884
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	55,528	55,159
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	135,561	137,463
Deferral of effects on net margin - Smith Energy Facility | BC Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	9,379	1,817
Deferral of effects on net margin - Smith Energy Facility | TA Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	129,299	130,786
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 16,123	$ 8,436
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	30 years