The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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The increase (decrease) during the reporting period in the value of assets that are created when regulatory agencies permits public utilities to defer costs (revenues) to the balance sheet. This element is a the increase (decrease) of regulatory assets and liabilities combined.

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Effective rate, including composite rate, of cost of financing additions to qualifying regulated assets, commonly called allowance for funds used during construction.

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Composite depreciation rate for public utility plants in service.

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Useful life of other equipment owned by public utility, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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Summary of significant accounting policies - Depreciation and Electric plant (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Depreciation
Depreciation expense	$ 278,452	$ 269,280	$ 242,822
Regulatory assets	$ 1,212,305	$ 1,008,790
Electric plant
Allowance for funds used during construction (as a percent)	4.03%	3.90%	4.00%
Steam production
Depreciation
Annual depreciation rates (as a percent)	13.77%	14.47%	2.58%
Steam production \| Minimum
Depreciation
Remaining life of the plant	20 years
Steam production \| Maximum
Depreciation
Remaining life of the plant	22 years
Nuclear production
Depreciation
Annual depreciation rates (as a percent)	2.17%	2.18%	1.93%
Nuclear production \| Minimum
Depreciation
Remaining life of the plant	12 years
Nuclear production \| Maximum
Depreciation
Remaining life of the plant	27 years
Hydro production
Depreciation
Remaining life of the plant	44 years
Annual depreciation rates (as a percent)	2.00%	2.00%	2.00%
Other production
Depreciation
Annual depreciation rates (as a percent)	2.68%	2.60%	2.61%
Other production \| Minimum
Depreciation
Remaining life of the plant	17 years
Other production \| Maximum
Depreciation
Remaining life of the plant	26 years
Transmission
Depreciation
Annual depreciation rates (as a percent)	2.75%	2.75%	2.75%
Transmission \| Minimum
Depreciation
Remaining life of the plant	12 years
Transmission \| Maximum
Depreciation
Remaining life of the plant	27 years
General \| Minimum
Depreciation
Remaining life of the plant	1 year
Annual depreciation rates (as a percent)	2.00%	2.00%	2.00%
General \| Maximum
Depreciation
Remaining life of the plant	43 years
Annual depreciation rates (as a percent)	33.33%	33.33%	33.33%
Wansley
Depreciation
Depreciation expense	$ 8,120
Wansley \| Depreciation expense
Depreciation
Deferred depreciation expense	165,013	$ 204,891
Regulatory assets	$ 361,784	$ 204,891