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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income tax expense	CAD 618	CAD 445	CAD 1,197	CAD 1,155
Income tax recovery	(138)	(117)	(286)	(287)
Net income	480	328	911	868
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	(1)	(1)	(2)	(3)
Recognition of net actuarial loss	38	48	77	97
Income before income tax expense	37	47	75	94
Income tax recovery	(10)	(13)	(20)	(26)
Net income	CAD 27	CAD 34	CAD 55	CAD 68