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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan and Other Postretirement Benefit Plans
Total other components of net periodic benefit (recovery) cost	$ (99)	$ (96)	$ (294)	$ (287)
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans
Current service cost (benefits earned by employees)	27	30	81	90
Interest cost on benefit obligation	113	110	338	329
Expected return on fund assets	(236)	(239)	(710)	(716)
Recognized net actuarial loss	20	29	61	86
Amortization of prior service costs	(1)	(1)	(1)	(2)
Total other components of net periodic benefit (recovery) cost	(104)	(101)	(312)	(303)
Net periodic benefit (recovery) cost	(77)	(71)	(231)	(213)
Other Postretirement Benefit Plans, Defined Benefit [Member]
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	5	14	14
Expected return on fund assets	0	0	0	0
Recognized net actuarial loss	1	0	3	2
Amortization of prior service costs	0	0	1	0
Total other components of net periodic benefit (recovery) cost	5	5	18	16
Net periodic benefit (recovery) cost	$ 8	$ 8	$ 27	$ 25