Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 898,117	$ 731,438
Total Regulatory Liabilities	816,229	566,399
Net Regulatory Assets	81,888	165,039
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	21,124	20,054
Deferral of effects on net margin | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	17,407	17,869
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	61,127	39,776
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	9,182	11,356
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	134,833	123,772
Asset retirement obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	142,368	130,901
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 334,645	220,111
Amortization Period	5 years
Natural gas hedges
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 93,930	0
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 1,613	2,560
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization Period	6 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 34,307	35,433
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	$ 34,555	35,328
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 36,734	35,232
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	$ 272,923	242,832
Depreciation expense | Other
Regulatory Assets and Liabilities
Total Regulatory Assets	37,329	38,396
Depreciation expense | Plant Wansley
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 149,641	0
Depreciation expense | Plant Vogtle
Regulatory Assets and Liabilities
Operating license expected extension period for Plant Vogtle	20 years
Operating license period	40 years
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	$ 55,702	55,430
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	130,367	126,813
Deferral of effects on net margin - Smith Energy Facility | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	144,161	148,620
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 2,398	$ 13,354
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	29 years