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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, 2022
Dec. 31, 2021
Dec. 31, 2020
Deferred tax assets:      
Non-capital loss, investment tax credits, currently non-deductible interest expenses, and financing costs $ 878,000 $ 761,666  
Pension and OPEB 16,845 46,580  
Environmental obligation 12,118 15,271  
Regulatory liabilities 156,285 166,939  
Other 61,917 64,460  
Total deferred income tax assets 1,125,165 1,054,916  
Less: valuation allowance (107,583) (27,471) $ (29,824)
Total deferred tax assets 1,017,582 1,027,445  
Deferred tax liabilities:      
Property, plant and equipment 846,331 782,829  
Outside basis differentials 315,581 412,665  
Regulatory accounts 303,059 300,072  
Other 33,834 30,471  
Total deferred tax liabilities 1,498,805 1,526,037  
Net deferred tax liabilities (481,223) (498,592)  
Deferred tax assets 84,416 31,595  
Deferred tax liabilities $ (565,639) $ (530,187)