Pensions and other benefits - Summary of DB Pension Plan Assets at Fair Value (Details) $ in Millions, $ in Millions	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	$ 12,862	$ 14,938
Currency Forward
Defined Benefit Plan Disclosure [Line Items]
Derivative notional amount	0	0
Derivative fair value		6
Forward starting swaps
Defined Benefit Plan Disclosure [Line Items]
Derivative notional amount	1,745	2,967	$ 2,400	$ 2,400
Derivative fair value	(81)	100
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	218	363
Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,305	1,936
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,156	1,437
Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	184	234
Public equities, Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	769	1,004
Public equities, U.S and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	5,195	6,536
Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	722	732
Fund value subject to redemption	595	613
Fund value not subject to redemption	127	119
Unfunded commitments	40	32
Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	744	263
Fund value subject to redemption	356	107
Fund value not subject to redemption	388	156
Unfunded commitments	356	814
Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	992	682
Fund value subject to redemption	160	152
Fund value not subject to redemption	832	530
Unfunded commitments	747	774
Derivative instruments
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	(81)	106
Absolute return, Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,658	1,621
Absolute return, Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		24
Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	4,116	3,322
Investments measured at NAV | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Public equities, Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Public equities, U.S and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	722	732
Investments measured at NAV | Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	744	263
Investments measured at NAV | Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	992	682
Investments measured at NAV | Derivative instruments
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV | Absolute return, Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,658	1,621
Investments measured at NAV | Absolute return, Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		24
Fair Value, Level 1
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	6,976	8,934
Fair Value, Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	218	363
Fair Value, Level 1 | Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	180	232
Fair Value, Level 1 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	432	569
Fair Value, Level 1 | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	182	230
Fair Value, Level 1 | Public equities, Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	769	1,004
Fair Value, Level 1 | Public equities, U.S and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	5,195	6,536
Fair Value, Level 1 | Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 1 | Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 1 | Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 1 | Derivative instruments
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 1 | Absolute return, Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 1 | Absolute return, Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		0
Fair Value, Level 2
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,770	2,682
Fair Value, Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 2 | Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,125	1,704
Fair Value, Level 2 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	724	868
Fair Value, Level 2 | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2	4
Fair Value, Level 2 | Public equities, Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 2 | Public equities, U.S and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 2 | Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 2 | Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 2 | Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Level 2 | Derivative instruments
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	(81)	106
Fair Value, Level 2 | Absolute return, Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 0	0
Fair Value, Level 2 | Absolute return, Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		$ 0