Pensions and Other Benefits - Summary of Defined Benefit Pension Plan Assets at Fair Value (Detail) - CAD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 12,349	$ 12,808
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	139	176
Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,382	2,087
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,734	1,215
Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	41	45
Public equity | Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,287	1,529
Public equity | U.S. and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	4,916	5,296
Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	697	622
Fund value subject to redemption	583	542
Fund value not subject to redemption	114	80
Unfunded commitments	38	53
Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	259	176
Fund value subject to redemption	130
Fund value not subject to redemption	129	176
Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	162
Unfunded commitments	608	0
Derivative Financial Instruments, Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Derivative, Notional Amount	1,226	0
Derivative Financial Instruments, Liabilities [Member]
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	(7)
Derivative, Fair Value, Net	7
Absolute return | Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,189	681
Absolute return | Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	286	515
Absolute return | Credit funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	32	252
Absolute return | Equity funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	232	214
Investments measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2,857	2,460
Investments measured at NAV [Member] | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV [Member] | Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV [Member] | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV [Member] | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV [Member] | Public equity | Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV [Member] | Public equity | U.S. and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Investments measured at NAV [Member] | Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	697	622
Investments measured at NAV [Member] | Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	259	176
Investments measured at NAV [Member] | Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	162
Investments measured at NAV [Member] | Derivative Financial Instruments, Liabilities [Member]
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0
Investments measured at NAV [Member] | Absolute return | Funds of hedge funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,189	681
Investments measured at NAV [Member] | Absolute return | Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	286	515
Investments measured at NAV [Member] | Absolute return | Credit funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	32	252
Investments measured at NAV [Member] | Absolute return | Equity funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	232	214
Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	6,576	6,886
Fair Value, Inputs, Level 1 [Member] | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	127	165
Fair Value, Inputs, Level 1 [Member] | Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	101	0
Fair Value, Inputs, Level 1 [Member] | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	128	0
Fair Value, Inputs, Level 1 [Member] | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	41	0
Fair Value, Inputs, Level 1 [Member] | Public equity | Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,287	1,467
Fair Value, Inputs, Level 1 [Member] | Public equity | U.S. and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	4,892	5,254
Fair Value, Inputs, Level 1 [Member] | Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 1 [Member] | Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 1 [Member] | Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0
Fair Value, Inputs, Level 1 [Member] | Derivative Financial Instruments, Liabilities [Member]
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0
Fair Value, Inputs, Level 1 [Member] | 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 [Member] | Absolute return | Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 1 [Member] | Absolute return | Credit funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 1 [Member] | Absolute return | Equity funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	2,916	3,462
Fair Value, Inputs, Level 2 [Member] | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	12	11
Fair Value, Inputs, Level 2 [Member] | Government bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,281	2,087
Fair Value, Inputs, Level 2 [Member] | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	1,606	1,215
Fair Value, Inputs, Level 2 [Member] | Mortgages
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	45
Fair Value, Inputs, Level 2 [Member] | Public equity | Canada
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	62
Fair Value, Inputs, Level 2 [Member] | Public equity | U.S. and international
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	24	42
Fair Value, Inputs, Level 2 [Member] | Real estate
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 2 [Member] | Infrastructure
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 2 [Member] | Private debt
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0
Fair Value, Inputs, Level 2 [Member] | Derivative Financial Instruments, Liabilities [Member]
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	(7)
Fair Value, Inputs, Level 2 [Member] | 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 [Member] | Absolute return | Multi-strategy funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 2 [Member] | Absolute return | Credit funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	0	0
Fair Value, Inputs, Level 2 [Member] | Absolute return | Equity funds
Defined Benefit Plan Disclosure [Line Items]
DB pension plans at fair values	$ 0	$ 0