Pensions and Other Benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Detail) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Other components of net periodic benefit recovery	$ (384)	$ (274)	$ (167)
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees)	120	103	106
Interest cost on benefit obligation	438	451	467
Expected return on fund assets	(955)	(893)	(846)
Recognized net actuarial loss	114	153	190
Amortization of prior service costs	(2)	(5)	(7)
Other components of net periodic benefit recovery	(405)	(294)	(196)
Net periodic benefit (recovery) cost	(285)	(191)	(90)
Other Post-retirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees)	12	12	11
Interest cost on benefit obligation	19	20	21
Expected return on fund assets	0	0	0
Recognized net actuarial loss	2	(1)	7
Amortization of prior service costs	0	1	1
Other components of net periodic benefit recovery	21	20	29
Net periodic benefit (recovery) cost	$ 33	$ 32	$ 40