Pensions and other benefits - Summary of DB Pension Plan Assets at Fair Value (Details) $ in Millions, $ in Billions	Dec. 31, 2021 CAD ($)	Jun. 30, 2021 USD ($)	May 21, 2021 USD ($)	Dec. 31, 2020 CAD ($)
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 14,938			$ 14,365
Currency Forward
Defined Benefit Plan Disclosure [Line Items]
Derivative notional amount	0			1,041
Derivative fair value	6			73
Forward starting swaps
Defined Benefit Plan Disclosure [Line Items]
Derivative notional amount	2,967	$ 2.4	$ 2.4	3,540
Derivative fair value	100			(2)
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	363			219
Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,936			1,983
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,437			1,835
Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	234			225
Public equities, Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,004			1,183
Public equities, U.S and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	6,536			5,899
Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	732			704
Fund value subject to redemption	613			580
Fund value not subject to redemption	119			124
Unfunded commitments	32			32
Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	263			199
Fund value subject to redemption	107			112
Fund value not subject to redemption	156			87
Unfunded commitments	814			491
Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	682			465
Fund value subject to redemption	152			154
Fund value not subject to redemption	530			311
Unfunded commitments	774			533
Derivative instruments
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	106			71
Absolute return, Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,621			1,560
Absolute return, Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	24			22
Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	3,322			2,950
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	732			704
Investments measured at NAV | Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	263			199
Investments measured at NAV | Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	682			465
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	1,621			1,560
Investments measured at NAV | Absolute return, Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	24			22
Fair Value, Level 1
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	8,934			8,468
Fair Value, Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	363			219
Fair Value, Level 1 | Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	232			284
Fair Value, Level 1 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	569			691
Fair Value, Level 1 | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	230			220
Fair Value, Level 1 | Public equities, Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,004			1,183
Fair Value, Level 1 | Public equities, U.S and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	6,536			5,871
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			0
Fair Value, Level 2
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2,682			2,947
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	1,704			1,699
Fair Value, Level 2 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	868			1,144
Fair Value, Level 2 | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	4			5
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			28
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	106			71
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			$ 0