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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, 2018
Dec. 31, 2017
Deferred tax assets:    
Non-capital loss, investment tax credits, currently non-deductible interest expenses, and financing costs $ 329,099 $ 328,679
Pension and OPEB 48,586 43,638
Acquisition-related costs 1,420 1,601
Environmental obligation 14,790 14,803
Reserves and other non-deductible costs 20,517 30,652
Regulatory liabilities 161,560 154,597
Financial derivatives 12,831  
Other 10,425 16,384
Total deferred income tax assets 599,228 597,961
Less valuation allowance (28,018) (19,951)
Total deferred tax assets 571,210 578,010
Deferred tax liabilities:    
Property, plant and equipment (653,962) (668,083)
Intangible assets (7,247) (7,157)
Outside basis in partnership (167,659) (125,519)
Regulatory accounts (113,758) (114,062)
Financial derivatives 0 (980)
Other (314) 0
Total deferred tax liabilities (942,940) (915,801)
Net deferred tax liabilities (371,730) (337,791)
Deferred tax assets 72,415 61,357
Deferred tax liabilities $ (444,145) $ (399,148)