Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 739,089	$ 763,512
Total Regulatory Liabilities	480,446	364,241
Net Regulatory Assets	258,643	399,271
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	18,993	12,692
Deferral of effects on net margin | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	18,023	18,485
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	53,057	50,144
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	12,081	14,256
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	121,443	114,453
Asset Retirement Obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	66,458	61,516
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 187,600	90,066
Amortization Period	5 years
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 2,791	2,629
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization Period	7 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 36,529	40,067
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	24 years
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 35,354	35,433
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 42,440	34,367
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	$ 247,125	245,932
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 38,752	39,820
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	$ 54,599	53,466
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	125,629	121,938
Deferral of effects on net margin - Smith Energy Facility | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	150,106	154,564
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 8,555	$ 37,925
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	30 years