Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 1,069,926	$ 1,103,633
Total Regulatory Liabilities	689,206	661,592
Net Regulatory Assets	380,720	442,041
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	6,045	29,975
Deferral of effects on net margin | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	14,942	15,404
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	95,616	99,987
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	198,101	186,757
Asset retirement obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	224,143	102,858
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 130,826	197,373
Amortization period, regulatory liability	5 years
Natural gas hedges
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 18,898	28,624
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 635	614
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization period, regulatory liability	2 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 20,801	21,587
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	19 years
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 20,841	24,699
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 49,977	45,749
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	$ 251,863	268,074
Depreciation expense | Plant Vogtle
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 31,634	32,702
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	$ 321,376	337,181
Deferred charges related to Vogtle Units No. 3 and No. 4 training costs | Deferred charges related to Vogtle Units No. 3 and No. 4 training costs
Regulatory Assets and Liabilities
Total Regulatory Assets	53,851	54,545
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	125,351	130,456
Deferral of effects on net margin | TA Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	120,382	124,840
Deferral of effects on net margin | BC Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	32,576	27,841
Inventory adjustments | TA Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	13,957	14,723
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 27,317	$ 21,236
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	29 years