Pensions and Other Benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Detail) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the year)	CAD 106	CAD 126	CAD 106
Interest cost on benefit obligation	467	463	477
Expected return on fund assets	(846)	(816)	(757)
Recognized net actuarial loss (gain)	190	265	190
Amortization of prior service costs	(7)	(6)	(68)
Net periodic benefit (recovery) cost	(90)	32	(52)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the year)	11	12	14
Interest cost on benefit obligation	21	21	23
Expected return on fund assets	0	0	0
Recognized net actuarial loss (gain)	7	2	(2)
Amortization of prior service costs	1	1	0
Net periodic benefit (recovery) cost	CAD 40	CAD 36	CAD 35