Pensions and Other Benefits - Net Periodic Benefit Cost (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Total other components of net periodic benefit (recovery) cost	$ (342)	$ (381)	$ (384)
Pension Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees)	140	107	120
Interest cost on benefit obligation	406	450	438
Expected return on fund assets	(945)	(947)	(955)
Recognized net actuarial loss	177	84	114
Amortization of prior service costs	(1)	(1)	(2)
Total other components of net periodic benefit (recovery) cost	(363)	(414)	(405)
Net periodic benefit (recovery) cost	(223)	(307)	(285)
Other Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees)	12	11	12
Interest cost on benefit obligation	17	20	19
Expected return on fund assets	0	0	0
Recognized net actuarial loss	4	12	2
Amortization of prior service costs	0	1	0
Total other components of net periodic benefit (recovery) cost	21	33	21
Net periodic benefit (recovery) cost	$ 33	$ 44	$ 33