Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss, Net of Tax (Detail) - CAD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net deferred and unrealized losses on derivatives and other	$ (62)	$ (89)
Amounts for defined benefit pension and other post-retirement plans not recognized in income	(2,094)	(1,761)
Accumulated other comprehensive loss	(2,043)	(1,741)	$ (1,799)
Net investment in U.S. subsidiaries
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss) on net investment	862	443
USD long-term debt designated as hedge of net investment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss) on net investment	$ (749)	$ (334)