Pensions and Other Benefits - Information About DB Pension Plans and Other Benefits (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in fund assets:
Fair value of fund assets at January 1	CAD 12,300
Employer contributions	57	CAD 90	CAD 88
Fair value of fund assets at December 31	12,196	12,300
Pension Plans, Defined Benefit [Member]
Change in projected benefit obligation:
Benefit obligation at January 1	11,194	11,360
Current service cost	106	126	106
Interest cost	467	463	477
Employee contributions	40	43
Benefits paid	(645)	(608)
Foreign currency changes	(7)	42
Plan amendments and other	6	(6)
Actuarial loss (gain)	238	(226)
Projected benefit obligation at December 31	11,399	11,194	11,360
Change in fund assets:
Fair value of fund assets at January 1	12,300	11,376
Actual return on fund assets	461	1,374
Employer contributions	48	81
Employee contributions	40	43
Benefits paid	(645)	(608)
Foreign currency changes	(8)	34
Fair value of fund assets at December 31	12,196	12,300	11,376
Funded status – plan surplus (deficit)	797	1,106
Other Postretirement Benefit Plans, Defined Benefit [Member]
Change in projected benefit obligation:
Benefit obligation at January 1	513	517
Current service cost	11	12	14
Interest cost	21	21	23
Employee contributions	1	1
Benefits paid	(31)	(34)
Foreign currency changes	0	4
Plan amendments and other	0	0
Actuarial loss (gain)	(5)	(8)
Projected benefit obligation at December 31	510	513	517
Change in fund assets:
Fair value of fund assets at January 1	6	7
Actual return on fund assets	(1)	(1)
Employer contributions	30	33
Employee contributions	1	1
Benefits paid	(31)	(34)
Foreign currency changes	0	0
Fair value of fund assets at December 31	5	6	CAD 7
Funded status – plan surplus (deficit)	CAD (505)	CAD (507)