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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense	CAD 680	CAD 470	CAD 1,877	CAD 1,625
Income tax (recovery) expense	(170)	(123)	(456)	(410)
Net income	510	347	1,421	1,215
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	(1)	(2)	(3)	(5)
Recognition of net actuarial loss	39	49	116	146
Income before income tax expense	38	47	113	141
Income tax (recovery) expense	(10)	(13)	(30)	(39)
Net income	CAD 28	CAD 34	CAD 83	CAD 102