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Income taxes - Tax Effect on Significant Portions of Deferred Tax Assets and Deferred Tax Liabilities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Deferred tax assets:      
Non-capital loss, investment tax credits, currently non-deductible interest expenses, and financing costs $ 1,030,801 $ 878,000  
Pension and OPEB 7,370 16,845  
Environmental obligation 11,692 12,118  
Regulatory liabilities 180,371 156,285  
Other 72,109 61,917  
Total deferred income tax assets 1,302,343 1,125,165  
Less: valuation allowance (97,344) (107,583) $ (27,471)
Total deferred tax assets 1,204,999 1,017,582  
Deferred tax liabilities:      
Property, plant and equipment 883,447 846,331  
Outside basis differentials 364,511 315,581  
Regulatory accounts 317,820 303,059  
Other 59,640 33,834  
Total deferred tax liabilities 1,625,418 1,498,805  
Net deferred tax liabilities (420,419) (481,223)  
Deferred tax assets 158,483 84,416  
Deferred tax liabilities $ (578,902) $ (565,639)