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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, 2021
Dec. 31, 2020
Deferred tax assets:    
Non-capital loss, investment tax credits, currently non-deductible interest expenses, and financing costs $ 761,666 $ 531,353
Pension and OPEB 46,580 66,826
Environmental obligation 15,271 16,145
Regulatory liabilities 166,939 168,054
Other 64,460 65,787
Total deferred income tax assets 1,054,916 848,165
Less: valuation allowance (27,471) (29,824)
Total deferred tax assets 1,027,445 818,341
Deferred tax liabilities:    
Property, plant and equipment 782,829 733,211
Outside basis differentials 412,665 406,429
Regulatory accounts 300,072 212,937
Other 30,471 12,528
Total deferred tax liabilities 1,526,037 1,365,105
Net deferred tax liabilities (498,592) (546,764)
Deferred tax assets 31,595 21,880
Deferred tax liabilities $ (530,187) $ (568,644)