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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, 2019
Dec. 31, 2018
Deferred tax assets:    
Non-capital loss, investment tax credits, currently non-deductible interest expenses, and financing costs $ 382,448 $ 329,099
Pension and OPEB 54,113 48,586
Environmental obligation 15,541 14,790
Regulatory liabilities 160,200 161,560
Other 59,103 45,193
Total deferred income tax assets 671,405 599,228
Less: valuation allowance (29,447) (28,018)
Total deferred tax assets 641,958 571,210
Deferred tax liabilities:    
Property, plant and equipment (707,185) (653,962)
Outside basis in partnership 235,063 167,659
Regulatory accounts 145,852 113,758
Other 14,811 7,561
Total deferred tax liabilities 1,102,911 942,940
Net deferred tax liabilities (460,953) (371,730)
Deferred tax assets 30,585 72,415
Deferred tax liabilities $ (491,538) $ (444,145)