Pensions and Other Benefits - Summary of DB Pension Plan Assets at Fair Value (Details) - CAD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 14,365	$ 13,319
Currency Forward
Defined Benefit Plan Disclosure [Line Items]
Derivative, Notional Amount	1,041	334
Derivative, Fair Value, Net	73	13
Bond Forward
Defined Benefit Plan Disclosure [Line Items]
Derivative, Notional Amount	3,540	3,269
Derivative, Fair Value, Net	(2)	(72)
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	219	112
Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,983	2,090
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,835	1,092
Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	225	164
Public equities, Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,183	1,351
Public equities, U.S and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	5,899	5,905
Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	704	724
Fund value subject to redemption	580	606
Fund value not subject to redemption	124	118
Unfunded commitments	32	35
Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	199	187
Fund value subject to redemption	112	119
Fund value not subject to redemption	87	68
Unfunded commitments	491	286
Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	465	313
Fund value subject to redemption	154	154
Fund value not subject to redemption	311	159
Unfunded commitments	533	392
Derivative instruments
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	71	(59)
Absolute return, Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,560	1,418
Absolute return, Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	22	22
Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2,950	2,664
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	704	724
Investments measured at NAV | Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	199	187
Investments measured at NAV | Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	465	313
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	1,560	1,418
Investments measured at NAV | Absolute return, Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	22	22
Fair Value, Level 1
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	8,468	8,011
Fair Value, Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	219	112
Fair Value, Level 1 | Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	284	233
Fair Value, Level 1 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	691	273
Fair Value, Level 1 | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	220	159
Fair Value, Level 1 | Public equities, Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,183	1,351
Fair Value, Level 1 | Public equities, U.S and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	5,871	5,883
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	2,947	2,644
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	1,699	1,857
Fair Value, Level 2 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,144	819
Fair Value, Level 2 | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	5	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	28	22
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	71	(59)
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