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, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the year)	CAD 103	CAD 106	CAD 126
Interest cost on benefit obligation	451	467	463
Expected return on fund assets	(893)	(846)	(816)
Recognized net actuarial loss (gain)	153	190	265
Amortization of prior service costs	(5)	(7)	(6)
Total other components of net periodic benefit (recovery) cost	(294)	(196)	(94)
Net periodic benefit (recovery) cost	(191)	(90)	32
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the year)	12	11	12
Interest cost on benefit obligation	20	21	21
Expected return on fund assets	0	0	0
Recognized net actuarial loss (gain)	(1)	7	2
Amortization of prior service costs	1	1	1
Total other components of net periodic benefit (recovery) cost	20	29	24
Net periodic benefit (recovery) cost	CAD 32	CAD 40	CAD 36