Pensions and Other Benefits - Summary of Defined Benefit Pension Plan Assets at Fair Value (Details) - CAD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 13,319	$ 12,349
Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2,664	2,857
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	112	139
Cash and cash equivalents | Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2,090	1,382
Government bonds | Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,092	1,734
Corporate bonds | Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	164	41
Mortgages | Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Public equities | Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,351	1,287
Public equities | Canada | Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Public equities | U.S. and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	5,905	4,916
Public equities | U.S. and international | Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	724	697
Fund value subject to redemption	606	583
Fund value not subject to redemption	118	114
Unfunded commitments	35	38
Real estate | Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	724	697
Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	187	259
Fund value subject to redemption	119	130
Fund value not subject to redemption	68	129
Infrastructure | Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	187	259
Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	313	162
Fund value subject to redemption	154	162
Fund value not subject to redemption	159
Unfunded commitments	392	608
Private debt | Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	313	162
Derivative instruments
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	(59)	(7)
Derivative instruments | Currency Forward
Defined Benefit Plan Disclosure [Line Items]
Derivative, Notional Amount	334	1,226
Derivative, Fair Value, Net	13	(7)
Derivative instruments | Bond Forward
Defined Benefit Plan Disclosure [Line Items]
Derivative, Notional Amount	3,269
Derivative, Fair Value, Net	72
Derivative instruments | Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Absolute return | Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,418	1,189
Absolute return | Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	22	286
Absolute return | Credit funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		32
Absolute return | Equity funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		232
Absolute return | Investments measured at NAV | Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,418	1,189
Absolute return | Investments measured at NAV | Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	22	286
Absolute return | Investments measured at NAV | Credit funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		32
Absolute return | Investments measured at NAV | Equity funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		232
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	8,011	6,576
Fair Value, Inputs, Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	112	127
Fair Value, Inputs, Level 1 | Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	233	101
Fair Value, Inputs, Level 1 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	273	128
Fair Value, Inputs, Level 1 | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	159	41
Fair Value, Inputs, Level 1 | Public equities | Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,351	1,287
Fair Value, Inputs, Level 1 | Public equities | U.S. and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	5,883	4,892
Fair Value, Inputs, Level 1 | Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 1 | Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 1 | Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 1 | Derivative instruments
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 1 | Absolute return | Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 1 | Absolute return | Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 1 | Absolute return | Credit funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		0
Fair Value, Inputs, Level 1 | Absolute return | Equity funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		0
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2,644	2,916
Fair Value, Inputs, Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	12
Fair Value, Inputs, Level 2 | Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,857	1,281
Fair Value, Inputs, Level 2 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	819	1,606
Fair Value, Inputs, Level 2 | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	5	0
Fair Value, Inputs, Level 2 | Public equities | Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 2 | Public equities | U.S. and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	22	24
Fair Value, Inputs, Level 2 | Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 2 | Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 2 | Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 2 | Derivative instruments
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	(59)	(7)
Fair Value, Inputs, Level 2 | Absolute return | Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 2 | Absolute return | Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 0	0
Fair Value, Inputs, Level 2 | Absolute return | Credit funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		0
Fair Value, Inputs, Level 2 | Absolute return | Equity funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values		$ 0