Pensions and Other Benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Total other components of net periodic benefit (recovery) cost	$ (95)	$ (68)	$ (191)	$ (135)
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees)	30	26	60	51
Interest cost on benefit obligation	109	113	219	226
Expected return on fund assets	(238)	(223)	(477)	(446)
Recognized net actuarial loss	28	38	57	76
Amortization of prior service costs	0	(1)	(1)	(2)
Total other components of net periodic benefit (recovery) cost	(101)	(73)	(202)	(146)
Net periodic benefit (recovery) cost	(71)	(47)	(142)	(95)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees)	3	3	6	6
Interest cost on benefit obligation	5	5	9	10
Expected return on fund assets	0	0	0	0
Recognized net actuarial loss	1	0	2	1
Amortization of prior service costs	0	0	0	0
Total other components of net periodic benefit (recovery) cost	6	5	11	11
Net periodic benefit (recovery) cost	$ 9	$ 8	$ 17	$ 17