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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pensions [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the period)	CAD 26	CAD 31	CAD 79	CAD 95
Interest cost on benefit obligation	117	116	350	347
Expected return on fund assets	(211)	(201)	(634)	(614)
Recognized net actuarial loss	48	66	143	198
Amortization of prior service costs	(2)	(2)	(5)	(5)
Net periodic (recovery) benefit cost	(22)	10	(67)	21
Other benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Current service cost (benefits earned by employees in the period)	2	3	8	9
Interest cost on benefit obligation	6	6	16	16
Expected return on fund assets	0	0	0	0
Recognized net actuarial loss	1	0	3	2
Amortization of prior service costs	0	0	0	0
Net periodic (recovery) benefit cost	CAD 9	CAD 9	CAD 27	CAD 27