Other comprehensive income (loss) and accumulated other comprehensive loss - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Unrealized foreign exchange (loss) gain on: Before tax amount
Translation of the net investment in U.S. subsidiaries, before tax	$ (316)	$ (118)	$ (251)
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries, before tax	25	136	288
Realized loss on derivatives designated as cash flow hedges recognized in income, before tax	10	10	12
Unrealized gain on cash flow hedges, before tax	38
Change in pension and other benefits actuarial gains and losses, before tax	1,286	(403)	(661)
Change in prior service pension and other benefit costs, before tax		(4)
Equity accounted investments, before tax	9	(1)	(2)
Other comprehensive (loss) income: Before tax amount	1,052	(380)	(614)
Unrealized foreign exchange gain (loss) on: Income tax (expense) recovery
Translation of the net investment in U.S. subsidiaries, tax	0	0	0
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries, tax	(3)	(18)	(38)
Realized loss on derivatives designated as cash flow hedges recognized in income, tax	(3)	(3)	(2)
Unrealized gain on cash flow hedges, tax	(9)
Change in pension and other benefits actuarial gains and losses, tax	(323)	108	175
Change in prior service pension and other benefit costs, tax		1
Equity accounted investments, tax	(3)	0	0
Other comprehensive (loss) income: Income tax (expense) recovery	(341)	88	135
Unrealized foreign exchange gain (loss) on: Net of tax amount
Translation of the net investment in U.S. subsidiaries, after tax	(316)	(118)	(251)
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries, after tax	22	118	250
Realized loss on derivatives designated as cash flow hedges recognized in income, after tax	7	7	10
Unrealized gain on cash flow hedges, after tax	29
Change in pension and other benefits actuarial gains and losses, after tax	963	(295)	(486)
Change in prior service pension and other benefit costs, after tax		(3)
Equity accounted investments, after tax	6	(1)	(2)
Other comprehensive income (loss)	$ 711	$ (292)	$ (479)