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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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 51	CAD 53
Interest cost on benefit obligation	113	116	226	233
Expected return on fund assets	(223)	(211)	(446)	(423)
Recognized net actuarial loss	38	47	76	95
Amortization of prior service costs	(1)	(1)	(2)	(3)
Net periodic benefit (recovery) cost	(47)	(23)	(95)	(45)
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	6	6
Interest cost on benefit obligation	5	5	10	10
Expected return on fund assets	0	0	0	0
Recognized net actuarial loss	0	1	1	2
Amortization of prior service costs	0	0	0	0
Net periodic benefit (recovery) cost	CAD 8	CAD 9	CAD 17	CAD 18