Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2022	Dec. 31, 2021
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 1,314,464	$ 1,008,790
Total Regulatory Liabilities	873,549	849,449
Net Regulatory Assets	440,915	159,341
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	35,905	22,197
Deferral of effects on net margin - Hawk Road Energy Facility | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	16,790	17,253
Deferral of effects on net margin - Hawk Road Energy Facility | Effingham Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	20,764	0
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	97,745	73,059
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	6,282	8,457
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	150,615	138,897
Asset retirement obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	0	164,256
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 352,115	359,799
Amortization Period	5 years
Natural gas hedges
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 192,026	63,994
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 1,307	1,537
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization Period	5 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 30,498	33,200
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	22 years
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 32,476	34,179
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 33,301	31,956
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	$ 411,072	335,231
Asset retirement obligations | Nuclear
Regulatory Assets and Liabilities
Total Regulatory Assets	65,148	0
Depreciation expense | Plant Vogtle
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 35,905	36,973
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	$ 366,770	204,891
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	54,488	55,857
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	135,434	131,556
Deferral of effects on net margin - Smith Energy Facility | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	138,216	142,675
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 11,156	$ 2,272
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	28 years