Pension and Other Benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan and Other Postretirement Benefit Plans
Total other components of net periodic benefit (recovery) cost	$ (95)	$ (86)	$ (286)	$ (257)
Defined Benefit Pension Plan
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	42	35	128	105
Interest cost on benefit obligation	88	102	264	305
Expected return on fund assets	(240)	(236)	(720)	(709)
Recognized net actuarial loss	52	44	155	132
Amortization of prior service costs (recoveries)	0	(1)	0	(1)
Total other components of net periodic benefit (recovery) cost	(100)	(91)	(301)	(273)
Net periodic benefit (recovery) cost	(58)	(56)	(173)	(168)
Other Post-retirement Benefit Plans
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	4	3	10	9
Interest cost on benefit obligation	4	4	12	13
Expected return on fund assets	0	0	0	0
Recognized net actuarial loss	1	1	3	3
Amortization of prior service costs (recoveries)	0	0	0	0
Total other components of net periodic benefit (recovery) cost	5	5	15	16
Net periodic benefit (recovery) cost	$ 9	$ 8	$ 25	$ 25