Pension and Other Benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan and Other Postretirement Benefit Plans
Total other components of net periodic benefit (recovery) cost	$ (95)	$ (85)
Defined Benefit Pension Plan
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	43	35
Interest cost on benefit obligation	88	102
Expected return on fund assets	(240)	(237)
Recognized net actuarial loss	52	44
Total other components of net periodic benefit (recovery) cost	(100)	(91)
Net periodic benefit (recovery) cost	(57)	(56)
Other Post-retirement Benefit Plans
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	3	3
Interest cost on benefit obligation	4	5
Expected return on fund assets	0	0
Recognized net actuarial loss	1	1
Total other components of net periodic benefit (recovery) cost	5	6
Net periodic benefit (recovery) cost	$ 8	$ 9