Summary of significant accounting policies - Depreciation and Electric plant (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Depreciation
Depreciation expense	$ 269,280	$ 242,822	$ 237,447
Electric plant
Allowance for funds used during construction (as a percent)	3.90%	4.00%	4.30%
Regulatory assets	$ 1,008,790	$ 731,438
Depreciation expense
Electric plant
Regulatory assets	$ 36,973	$ 38,396
Steam production
Depreciation
Annual depreciation rates (as a percent)	14.47%	2.58%	2.61%
Steam production | Minimum
Depreciation
Remaining Useful Life Range in years	1 year
Steam production | Maximum
Depreciation
Remaining Useful Life Range in years	23 years
Nuclear production
Depreciation
Annual depreciation rates (as a percent)	2.18%	1.93%	1.94%
Nuclear production | Minimum
Depreciation
Remaining Useful Life Range in years	13 years
Nuclear production | Maximum
Depreciation
Remaining Useful Life Range in years	28 years
Hydro production
Depreciation
Remaining Useful Life Range in years	45 years
Annual depreciation rates (as a percent)	2.00%	2.00%	2.00%
Other production
Depreciation
Annual depreciation rates (as a percent)	2.60%	2.61%	2.61%
Other production | Minimum
Depreciation
Remaining Useful Life Range in years	18 years
Other production | Maximum
Depreciation
Remaining Useful Life Range in years	27 years
Transmission
Depreciation
Annual depreciation rates (as a percent)	2.75%	2.75%	2.75%
Transmission | Minimum
Depreciation
Remaining Useful Life Range in years	13 years
Transmission | Maximum
Depreciation
Remaining Useful Life Range in years	28 years
General | Minimum
Depreciation
Remaining Useful Life Range in years	1 year
Annual depreciation rates (as a percent)	2.00%	2.00%	2.00%
General | Maximum
Depreciation
Remaining Useful Life Range in years	44 years
Annual depreciation rates (as a percent)	33.33%	33.33%	33.33%
Wansley | Depreciation expense
Electric plant
Regulatory assets	$ 204,891	$ 0