Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 803,584	$ 731,438
Total Regulatory Liabilities	722,972	566,399
Net Regulatory Assets	80,612	165,039
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	18,755	20,054
Deferral of effects on net margin | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	17,561	17,869
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	47,386	39,776
Amortization of financing leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	9,907	11,356
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	130,768	123,772
Asset Retirement Obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	161,049	130,901
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 299,082	220,111
Amortization Period	5 years
Natural gas hedges
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 36,723	0
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 1,741	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	$ 33,293	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	$ 34,930	35,328
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 34,440	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	$ 236,744	242,832
Depreciation expense | Other
Regulatory Assets and Liabilities
Total Regulatory Assets	37,684	38,396
Depreciation expense | Plant Wansley
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 94,075	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	$ 55,760	55,430
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	129,182	126,813
Deferral of effects on net margin - Smith Energy Facility | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	145,647	148,620
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 1,829	$ 13,354
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	29 years