Pensions and Other Benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Details) - CAD CAD in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the period)	CAD 25	CAD 27
Interest cost on benefit obligation	113	117
Expected return on fund assets	(223)	(212)
Recognized net actuarial loss	38	48
Amortization of prior service costs	(1)	(2)
Net periodic benefit (recovery) cost	(48)	(22)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the period)	3	3
Interest cost on benefit obligation	5	5
Expected return on fund assets	0	0
Recognized net actuarial loss	1	1
Amortization of prior service costs	0	0
Net periodic benefit (recovery) cost	CAD 9	CAD 9