Pensions and Other Benefits - Net Periodic Benefit Cost for DB Pension Plans and Other Benefits (Details) - CAD CAD in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the period)	CAD 26	CAD 32	CAD 53	CAD 64
Interest cost on benefit obligation	116	116	233	231
Expected return on fund assets	(211)	(212)	(423)	(413)
Recognized net actuarial loss	47	66	95	132
Amortization of prior service costs	(1)	(2)	(3)	(3)
Net periodic benefit (recovery) cost	(23)	0	(45)	11
Other benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the period)	3	3	6	6
Interest cost on benefit obligation	5	5	10	10
Expected return on fund assets	0	0	0	0
Recognized net actuarial loss	1	1	2	2
Amortization of prior service costs	0	0	0	0
Net periodic benefit (recovery) cost	CAD 9	CAD 9	CAD 18	CAD 18