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, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred losses on settled hedge instruments	CAD (3)	CAD (11)
Unrealized effective losses on cash flow hedges	(99)	(89)
Amounts for defined benefit pension and other post-retirement plans not recognized in income (Note 20)	(1,822)	(1,504)
Equity accounted investments	(2)	(2)
Accumulated other comprehensive loss	(1,799)	(1,477)
Net Investment in U.S. Subsidiaries [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss)	738	870
Long-term Debt Designated as Hedge of Net Investment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss)	CAD (611)	CAD (741)