Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 1,088,130	$ 1,008,790
Total Regulatory Liabilities	898,095	849,449
Net Regulatory Assets	190,035	159,341
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	22,371	22,197
Deferral of effects on net margin - Hawk Road Energy Facility | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	17,098	17,253
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	81,492	73,059
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	7,732	8,457
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	142,817	138,897
Asset retirement obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	97,746	164,256
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 353,433	359,799
Amortization Period	5 years
Natural gas hedges
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 173,946	63,994
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 1,460	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	$ 32,199	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	$ 33,611	34,179
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 45,834	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	$ 340,209	335,231
Depreciation expense | Plant Vogtle
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 36,617	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	$ 260,463	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	55,105	55,857
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	132,756	131,556
Deferral of effects on net margin - Smith Energy Facility | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	141,189	142,675
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 10,147	$ 2,272
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	28 years