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Income Taxes - Summary of Major Components of Current Income Tax Expense from Continuing Operations (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Major components of tax expense (income) [abstract]    
Ongoing operations $ 64.5 $ 34.4
Transition tax related to US tax reform (10.0) 31.2
Disposition of subsidiary   124.1
Reorganization of corporate structure   3.2
Total current income tax expense $ 54.5 $ 192.9