Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss, Net of Tax (Details) - CAD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss) on net investment	$ 611	$ 862
Net deferred losses on derivatives and other	(54)	(62)
Amounts for defined benefit pension and other post-retirement plans not recognized in income	(2,580)	(2,094)
Accumulated other comprehensive loss	(2,522)	(2,043)	$ (1,741)
Net Investment Hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss) on net investment	$ (499)	$ (749)