Other comprehensive (loss) income and accumulated other comprehensive (loss) income - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	$ (705)	$ 2,309	$ 1,052
Income tax (expense) recovery	(4)	(115)	(341)
Net of tax amount	(709)	2,194	711
Realized loss on derivatives designated as cash flow hedges recognized in income
Net of tax amount	(32)	44
Unrealized Gain on Cash Flow Hedges [Abstract]
Net of tax amount	(741)	2,238
Translation of the net investment in U.S. subsidiaries and equity method investees
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	(840)	2,099	(316)
Income tax (expense) recovery	0	0	0
Net of tax amount	(840)	2,099	(316)
Translation of the U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries and equity method investees (Note 18)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	194	(471)	25
Income tax (expense) recovery	(22)	59	(3)
Net of tax amount	172	(412)	22
Derivatives
Realized loss on derivatives designated as cash flow hedges recognized in income
Before tax amount	7	6	10
Income tax (expense) recovery	(2)	(2)	(3)
Net of tax amount	5	4	7
Unrealized Gain on Cash Flow Hedges [Abstract]
Before tax amount			38
Income tax (expense) recovery			(9)
Net of tax amount			29
Change in pension and other benefits actuarial gains and losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	(57)	706	1,286
Income tax (expense) recovery	16	(182)	(323)
Net of tax amount	(41)	524	963
Change in prior service pension and other benefit costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	(16)	(26)
Income tax (expense) recovery	4	7
Net of tax amount	(12)	(19)
Equity accounted investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	7	(5)	9
Income tax (expense) recovery	0	3	(3)
Net of tax amount	$ 7	$ (2)	$ 6