Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 760,599	$ 731,438
Total Regulatory Liabilities	604,032	566,399
Net Regulatory Assets	156,567	165,039
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	19,143	20,054
Deferral of effects on net margin | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	17,715	17,869
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	36,510	39,776
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	10,632	11,356
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	126,704	123,772
Asset Retirement Obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	132,148	130,901
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 259,240	220,111
Amortization Period	5 years
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 1,940	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	$ 34,363	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	$ 35,129	35,328
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 39,631	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	$ 239,755	242,832
Depreciation expense | Other
Regulatory Assets and Liabilities
Total Regulatory Assets	38,040	38,396
Depreciation expense | Plant Wansley
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 38,514	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	$ 55,706	55,430
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	127,997	126,813
Deferral of effects on net margin - Smith Energy Facility | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	147,133	148,620
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 4,331	$ 13,354
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	29 years