Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss, Net of Tax (Details) - CAD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss) on net investment	$ 493	$ 611
Net deferred losses on derivatives and other	(48)	(54)
Amounts for defined benefit pension and other post-retirement plans not recognized in income	(2,878)	(2,580)
Accumulated other comprehensive loss	(2,814)	(2,522)	$ (2,043)
Net Investment Hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss) on net investment	$ (381)	$ (499)