Other comprehensive income (loss) and Accumulated other comprehensive income (loss) - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	$ 3,517	$ (705)	$ 2,309
Income tax (expense) recovery	(219)	(4)	(115)
Net of tax amount	3,298	(709)	2,194
Realized loss on derivatives designated as cash flow hedges recognized in income
Net of tax amount	(40)	(32)
Unrealized Gain on Cash Flow Hedges [Abstract]
Net of tax amount	3,258	(741)
Translation of net investment in U.S. subsidiaries and equity method investees
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	2,920	(840)	2,099
Income tax (expense) recovery	0	0	0
Net of tax amount	2,920	(840)	2,099
Translation of U.S. dollar-denominated long-term debt designated as a hedge of the net investment in U.S. subsidiaries and equity method investees (Note 17)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	(380)	194	(471)
Income tax (expense) recovery	36	(22)	59
Net of tax amount	(344)	172	(412)
Derivatives
Realized loss on derivatives designated as cash flow hedges recognized in income
Before tax amount	6	7	6
Income tax (expense) recovery	(1)	(2)	(2)
Net of tax amount	5	5	4
Unrealized Gain on Cash Flow Hedges [Abstract]
Before tax amount			706
Income tax (expense) recovery			(182)
Net of tax amount			524
Change in pension and other benefits actuarial gains and losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	990	(57)	(26)
Income tax (expense) recovery	(257)	16	7
Net of tax amount	733	(41)	(19)
Change in prior service pension and other benefit costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	(11)	(16)
Income tax (expense) recovery	3	4
Net of tax amount	(8)	(12)
Equity accounted investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before tax amount	(8)	7	(5)
Income tax (expense) recovery	0	0	3
Net of tax amount	$ (8)	$ 7	$ (2)