Pensions and other benefits - Summary of DB Pension Plan Assets at Fair Value (Details) $ in Millions, $ in Billions	12 Months Ended
	Dec. 31, 2023 CAD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2021 CAD ($)	Jun. 30, 2021 USD ($)	May 21, 2021 USD ($)
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 13,478	$ 12,867	$ 14,943
Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	13,472	12,862	$ 14,938
Forward starting swaps
Defined Benefit Plan Disclosure [Line Items]
Derivative notional amount	1,396	1,745		$ 2.4	$ 2.4
Derivative fair value	116	(81)
Cash and cash equivalents | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	297	218
Government bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2,111	1,305
Corporate bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,642	1,156
Mortgages | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	206	184
Mortgage-backed and Asset-backed Securities | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	123
Public equities, Canada | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	534	769
Public equities, U.S and international | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	4,293	5,195
Real estate
Defined Benefit Plan Disclosure [Line Items]
Fund value subject to redemption	480	595
Fund value not subject to redemption	83	127
Unfunded commitments	$ 166	40
Investment redemption, notice period	90 days
Real estate | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 563	722
Infrastructure
Defined Benefit Plan Disclosure [Line Items]
Fund value subject to redemption	493	356
Fund value not subject to redemption	468	388
Unfunded commitments	$ 220	356
Investment redemption, notice period	90 days
Infrastructure | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 961	744
Private debt
Defined Benefit Plan Disclosure [Line Items]
Fund value subject to redemption	124	160
Fund value not subject to redemption	1,004	832
Unfunded commitments	$ 540	747
Investment redemption, notice period	90 days
Private debt | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 1,128	992
Derivative instruments | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 116	(81)
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	$ 1,498	1,658
Investments measured at NAV | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	4,150	4,116
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
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	563	722
Investments measured at NAV | Infrastructure | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	961	744
Investments measured at NAV | Private debt | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,128	992
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	1,498	1,658
Quoted prices in active markets for identical assets (Level 1) | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	6,185	6,976
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	297	218
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	211	180
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	644	432
Quoted prices in active markets for identical assets (Level 1) | Mortgages | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	206	182
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
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	534	769
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	4,293	5,195
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	3,137	1,770
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	1,900	1,125
Significant other observable inputs (Level 2) | Corporate bonds | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	998	724
Significant other observable inputs (Level 2) | Mortgages | Pensions
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	2
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	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	116	(81)
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