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Income taxes - Schedule of Tax Effect on Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Deferred tax assets:      
Non-capital loss, investment tax credits, currently non-deductible interest expenses, and financing costs $ 781,761 $ 714,476  
Outside basis in renewable energy assets 140,203 0  
Pension and OPEB 515 7,327  
Environmental obligation 9,161 8,821  
Regulatory liabilities 164,402 180,371  
Other 57,680 51,333  
Total deferred income tax assets 1,153,722 962,328  
Less: valuation allowance (281,894) (5,562) $ (5,876)
Total deferred tax assets 871,828 956,766  
Deferred tax liabilities:      
Property, plant and equipment 866,529 810,025  
Pension and OPEB 9,148 0  
Outside basis differentials 165,010 191,990  
Regulatory accounts 329,343 317,829  
Other 67,753 51,610  
Total deferred tax liabilities 1,437,783 1,371,454  
Net deferred tax liabilities (565,955) (414,688)  
Deferred tax assets 11,218 151,576  
Deferred tax liabilities $ (577,173) $ (566,264)