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Retirement Benefit Plans - Summary of Actuarial Valuation of Plans (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [line items]    
Interest income expense $ (6) $ (12)
Defined Benefit Pension Plans [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Funding surplus (deficit) 298 286
Net accrued retirement benefit asset (liability) 164 242
Defined Benefit Pension Plans [member] | Defined benefit obligation [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Balance at beginning of year 2,224 2,106
Current service cost 50 48
Past service costs arising from plan improvements   10
Benefits paid (139) (153)
Interest income expense 73 79
Obligation experience adjustments 26 27
Effect from change in financial assumptions (127) 119
Effect from change in demographic assumptions 4  
Changes in foreign exchange rates 14 (12)
Balance at end of year 2,125 2,224
Defined Benefit Pension Plans [member] | Fair value of plan assets [Member]    
Disclosure of net defined benefit liability (asset) [line items]    
Balance at beginning of year 2,510 2,342
Benefits paid (139) (153)
Interest income expense 82 88
Return on plan assets, excluding amounts included in interest income (84) 212
Contributions by the employer 42 31
Changes in foreign exchange rates 12 (10)
Balance at end of year 2,423 2,510
Defined Benefit Pension Plans [member] | Less effect of the asset ceiling [Member]    
Disclosure of net defined benefit liability (asset) [line items]    
Balance at beginning of year 44 58
Interest on asset ceiling 1 3
Change in asset ceiling 89 (17)
Balance at end of year 134 44
Defined Benefit Pension Plans [member] | Represented by [Member]    
Disclosure of net defined benefit liability (asset) [line items]    
Pension assets (Note 14) 254 339
Accrued retirement benefit liability (90) (97)
Net accrued retirement benefit asset (liability) 164 242
Non-Pension Post-Retirement Benefit [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Funding surplus (deficit) (392) (455)
Net accrued retirement benefit asset (liability) (392) (455)
Non-Pension Post-Retirement Benefit [member] | Defined benefit obligation [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Balance at beginning of year 455 538
Current service cost 19 24
Benefits paid (19) (23)
Interest income expense 17 22
Obligation experience adjustments (30) (22)
Effect from change in financial assumptions (35) 25
Effect from change in demographic assumptions (20) (104)
Changes in foreign exchange rates 5 (5)
Balance at end of year 392 455
Non-Pension Post-Retirement Benefit [member] | Fair value of plan assets [Member]    
Disclosure of net defined benefit liability (asset) [line items]    
Benefits paid (19) (23)
Contributions by the employer 19 23
Non-Pension Post-Retirement Benefit [member] | Represented by [Member]    
Disclosure of net defined benefit liability (asset) [line items]    
Accrued retirement benefit liability (392) (455)
Net accrued retirement benefit asset (liability) $ (392) $ (455)