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Components of Net Periodic Benefit Cost and Changes in Amounts Recorded in Accumulated Other Comprehensive Income (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Total recorded in other comprehensive income $ 522 $ (402) $ (120)
Pension benefits      
Defined Benefit Plan Disclosure [Line Items]      
Current service cost 228 239 217
Interest cost 324 302 313
Expected return on plan assets (349) (402) (408)
Amortization of prior service cost   4 10
Amortization of actuarial loss (gain) 149 175 176
Net periodic benefit cost 352 318 308
Net actuarial loss (gain) 288 (116) 123
Amortization of net actuarial (loss) gain included in net periodic benefit cost (149) (175) (176)
Prior service costs 283    
Amortization of prior service cost included in net periodic benefit cost   (4) (10)
Total recorded in other comprehensive income 422 (295) (63)
Total recorded in net periodic benefit cost and other comprehensive income, before tax 774 23 245
Other postretirement benefits      
Defined Benefit Plan Disclosure [Line Items]      
Current service cost 16 17 16
Interest cost 20 22 23
Amortization of actuarial loss (gain) (1) 6 8
Net periodic benefit cost 35 45 47
Net actuarial loss (gain) 99 (101) (49)
Amortization of net actuarial (loss) gain included in net periodic benefit cost 1 (6) (8)
Total recorded in other comprehensive income 100 (107) (57)
Total recorded in net periodic benefit cost and other comprehensive income, before tax $ 135 $ (62) $ (10)