Other comprehensive income (loss) and accumulated other comprehensive loss - Components of Accumulated Other Comprehensive Loss, Net of Tax (Details) - CAD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net deferred losses on derivatives	$ (4)	$ (40)
Amounts for defined benefit pension and other post-retirement plans not recognized in income	(1,915)	(2,878)
Equity accounted investments	(2)	(8)
Accumulated other comprehensive loss	(2,103)	(2,814)	$ (2,522)
Net Investment Hedge | Foreign Exchange Contract 1
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss) on net investment	177	493
Net Investment Hedge | Foreign Exchange Contract 2
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized foreign exchange gain (loss) on net investment	$ (359)	$ (381)