Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss, Net of Tax (Detail) - CAD CAD in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net deferred and unrealized losses on derivatives	CAD (88)	CAD (102)
Amounts for defined benefit pension and other post-retirement plans not recognized in income	(1,761)	(1,822)
Equity accounted investments	(1)	(2)
Accumulated other comprehensive loss	(1,741)	(1,799)	CAD (1,477)
Net investment in U.S. subsidiaries
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss) on net investment	443	738
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	CAD (334)	CAD (611)