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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan and Other Postretirement Benefit Plans
Total other components of net periodic benefit (recovery) cost	$ (86)	$ (99)	$ (257)	$ (294)
Defined Benefit Pension Plan
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	35	27	105	81
Interest cost on benefit obligation	102	113	305	338
Expected return on fund assets	(236)	(236)	(709)	(710)
Recognized net actuarial loss	44	20	132	61
Amortization of prior service costs	(1)	(1)	(1)	(1)
Total other components of net periodic benefit (recovery) cost	(91)	(104)	(273)	(312)
Net periodic benefit (recovery) cost	(56)	(77)	(168)	(231)
Other Post-retirement Benefit Plans
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	3	3	9	9
Interest cost on benefit obligation	4	4	13	14
Expected return on fund assets	0	0	0	0
Recognized net actuarial loss	1	1	3	3
Amortization of prior service costs	0	0	0	1
Total other components of net periodic benefit (recovery) cost	5	5	16	18
Net periodic benefit (recovery) cost	$ 8	$ 8	$ 25	$ 27