Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Amounts in Pension and Post-Retirement Defined Benefit Plans Reclassified from AOCL (Details) - CAD CAD in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense	CAD 579	CAD 710
Income tax recovery	(148)	(170)
Net income	431	540
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	(1)	(2)
Recognition of net actuarial loss	39	49
Income before income tax expense	38	47
Income tax recovery	(10)	(13)
Net income	CAD 28	CAD 34