Summary of significant accounting policies - Regulatory assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 1,008,790	$ 731,438
Total Regulatory Liabilities	849,449	566,399
Net Regulatory Assets	159,341	165,039
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	22,197	20,054
Deferral of effects on net margin | Hawk Road Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Liabilities	17,253	17,869
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	73,059	39,776
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	8,457	11,356
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	138,897	123,772
Nuclear
Regulatory Assets and Liabilities
Total Regulatory Liabilities	164,256	130,901
Revenue deferral plan
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 359,799	220,111
Amortization period, other regulatory liabilities	5 years
Natural gas hedge
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 63,994	0
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 1,537	2,560
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory liabilities	5 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 33,200	35,433
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	22 years
Amortization on financing leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 34,179	35,328
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 31,956	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	$ 335,231	242,832
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	36,973	38,396
Depreciation expense | Wansley
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 204,891	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	$ 55,857	55,430
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	131,556	126,813
Deferral of effects on net margin | Smith Energy Facility
Regulatory Assets and Liabilities
Total Regulatory Assets	142,675	148,620
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 2,272	$ 13,354
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	28 years