Pensions and other benefits - Summary of DB Pension Plan Assets at Fair Value (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 14,598	$ 13,478	$ 12,867
Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	14,592	13,472	$ 12,862
Forward starting swaps
Defined Benefit Plan Disclosure [Line Items]
Derivative notional amount	555	1,396
Derivative fair value	2	116
Cash and cash equivalents | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	324	297
Government bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2,733	2,111
Corporate bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,981	1,642
Mortgages | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	194	206
Mortgage-backed and asset-backed securities | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	356	123
Public equities, Canada | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	482	534
Public equities, U.S and international | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	3,997	4,293
Real estate
Defined Benefit Plan Disclosure [Line Items]
Fund value subject to redemption	435	480
Fund value not subject to redemption	86	83
Unfunded commitments	$ 309	166
Investment redemption, notice period	90 days
Real estate | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 521	563
Infrastructure
Defined Benefit Plan Disclosure [Line Items]
Fund value subject to redemption	606	493
Fund value not subject to redemption	588	468
Unfunded commitments	$ 205	220
Investment redemption, notice period	90 days
Infrastructure | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 1,194	961
Private debt
Defined Benefit Plan Disclosure [Line Items]
Fund value subject to redemption	115	124
Fund value not subject to redemption	1,031	1,004
Unfunded commitments	$ 764	540
Investment redemption, notice period	90 days
Private debt | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 1,146	1,128
Derivative instruments | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ (9)	116
Absolute return | Minimum
Defined Benefit Plan Disclosure [Line Items]
Investment redemption, notice period	30 days
Absolute return | Maximum
Defined Benefit Plan Disclosure [Line Items]
Investment redemption, notice period	120 days
Absolute return, Funds of hedge funds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 1,673	1,498
Investments measured at NAV | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	4,534	4,150
Investments measured at NAV | Cash and cash equivalents | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Government bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Corporate bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Mortgages | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Mortgage-backed and asset-backed securities | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Public equities, Canada | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Public equities, U.S and international | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Real estate | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	521	563
Investments measured at NAV | Infrastructure | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,194	961
Investments measured at NAV | Private debt | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,146	1,128
Investments measured at NAV | Derivative instruments | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Absolute return, Funds of hedge funds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,673	1,498
Quoted prices in active markets for identical assets (Level 1) | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	5,879	6,185
Quoted prices in active markets for identical assets (Level 1) | Cash and cash equivalents | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	324	297
Quoted prices in active markets for identical assets (Level 1) | Government bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	192	211
Quoted prices in active markets for identical assets (Level 1) | Corporate bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	690	644
Quoted prices in active markets for identical assets (Level 1) | Mortgages | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	194	206
Quoted prices in active markets for identical assets (Level 1) | Mortgage-backed and asset-backed securities | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Quoted prices in active markets for identical assets (Level 1) | Public equities, Canada | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	482	534
Quoted prices in active markets for identical assets (Level 1) | Public equities, U.S and international | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	3,997	4,293
Quoted prices in active markets for identical assets (Level 1) | Real estate | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Quoted prices in active markets for identical assets (Level 1) | Infrastructure | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Quoted prices in active markets for identical assets (Level 1) | Private debt | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Quoted prices in active markets for identical assets (Level 1) | Derivative instruments | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Quoted prices in active markets for identical assets (Level 1) | Absolute return, Funds of hedge funds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Significant other observable inputs (Level 2) | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	4,179	3,137
Significant other observable inputs (Level 2) | Cash and cash equivalents | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Significant other observable inputs (Level 2) | Government bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2,541	1,900
Significant other observable inputs (Level 2) | Corporate bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,291	998
Significant other observable inputs (Level 2) | Mortgages | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Significant other observable inputs (Level 2) | Mortgage-backed and asset-backed securities | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	356	123
Significant other observable inputs (Level 2) | Public equities, Canada | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Significant other observable inputs (Level 2) | Public equities, U.S and international | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Significant other observable inputs (Level 2) | Real estate | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Significant other observable inputs (Level 2) | Infrastructure | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Significant other observable inputs (Level 2) | Private debt | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Significant other observable inputs (Level 2) | Derivative instruments | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	(9)	116
Significant other observable inputs (Level 2) | Absolute return, Funds of hedge funds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 0	$ 0