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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, 2020
Dec. 31, 2019
Deferred tax assets:    
Non-capital loss, investment tax credits, currently non-deductible interest expenses, and financing costs $ 531,353 $ 382,448
Pension and OPEB 66,826 54,113
Environmental obligation 16,145 15,541
Regulatory liabilities 168,054 160,200
Other 65,787 59,103
Total deferred income tax assets 848,165 671,405
Less: valuation allowance (29,824) (29,447)
Total deferred tax assets 818,341 641,958
Deferred tax liabilities:    
Property, plant and equipment 733,211 707,185
Outside basis in partnership 406,429 235,063
Regulatory accounts 212,937 145,852
Other 12,528 14,811
Total deferred tax liabilities 1,365,105 1,102,911
Net deferred tax liabilities (546,764) (460,953)
Deferred tax assets 21,880 30,585
Deferred tax liabilities $ (568,644) $ (491,538)