Pensions and Other Benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Other components of net periodic benefit (recovery) cost	$ (381)	$ (384)	$ (274)
Pension Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees)	107	120	103
Interest cost on benefit obligation	450	438	451
Expected return on fund assets	(947)	(955)	(893)
Recognized net actuarial loss	84	114	153
Amortization of prior service costs	(1)	(2)	(5)
Other components of net periodic benefit (recovery) cost	(414)	(405)	(294)
Net periodic benefit (recovery) cost	(307)	(285)	(191)
Other Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees)	11	12	12
Interest cost on benefit obligation	20	19	20
Expected return on fund assets	0	0	0
Recognized net actuarial loss	12	2	(1)
Amortization of prior service costs	1	0	1
Other components of net periodic benefit (recovery) cost	33	21	20
Net periodic benefit (recovery) cost	$ 44	$ 33	$ 32