Pensions and Other Benefits - Information About DB Pension Plans and Other Benefits (Detail) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in fund assets:
Fair value of fund assets, Beginning	$ 12,808
Fair value of fund assets, Ending	12,349	$ 12,808
Pension Plans, Defined Benefit [Member]
Change in projected benefit obligation:
Benefit obligation at January 1	11,679	11,399
Current service cost	120	103	$ 106
Interest cost	438	451	467
Employee contributions	47	44
Benefits paid	(640)	(648)
Foreign currency changes	20	(15)
Plan amendments and other	0	1
Actuarial (gain) loss	(292)	344
Projected benefit obligation at December 31	11,372	11,679	11,399
Change in fund assets:
Fair value of fund assets, Beginning	12,808	12,196
Actual return on fund assets	82	1,183
Employer contributions	36	46
Employee contributions	47	44
Benefits paid	(640)	(648)
Foreign currency changes	16	(13)
Fair value of fund assets, Ending	12,349	12,808	12,196
Funded status - plan surplus (deficit)	977	1,129
Other Post-retirement Benefit Plans, Defined Benefit [Member]
Change in projected benefit obligation:
Benefit obligation at January 1	518	510
Current service cost	12	12	11
Interest cost	19	20	21
Employee contributions	1	1
Benefits paid	(33)	(35)
Foreign currency changes	2	(3)
Plan amendments and other	0	0
Actuarial (gain) loss	(18)	13
Projected benefit obligation at December 31	501	518	510
Change in fund assets:
Fair value of fund assets, Beginning	4	5
Actual return on fund assets	0	(1)
Employer contributions	32	34
Employee contributions	1	1
Benefits paid	(33)	(35)
Foreign currency changes	0	0
Fair value of fund assets, Ending	4	4	$ 5
Funded status - plan surplus (deficit)	$ (497)	$ (514)