Pensions and Other Benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Details) - CAD CAD in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the period)	CAD 26	CAD 26	CAD 77	CAD 79
Interest cost on benefit obligation	112	117	338	350
Expected return on fund assets	(223)	(211)	(669)	(634)
Recognized net actuarial loss	38	48	114	143
Amortization of prior service costs	(1)	(2)	(3)	(5)
Net periodic (recovery) benefit cost	(48)	(22)	(143)	(67)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the period)	3	2	9	8
Interest cost on benefit obligation	5	6	15	16
Expected return on fund assets	0	0	0	0
Recognized net actuarial loss	1	1	2	3
Amortization of prior service costs	0	0	0	0
Net periodic (recovery) benefit cost	CAD 9	CAD 9	CAD 26	CAD 27