Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Amounts in Pension and Post-retirement Defined Benefit Plans Reclassified from AOCL (Details) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income tax expense (recovery)	$ 211	$ 199	$ 474	$ 443
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	(1)	(1)	0	(2)
Recognition of net actuarial loss	21	29	62	88
Total before income tax	20	28	62	86
Income tax expense (recovery)	(5)	(9)	(15)	(24)
Total net of income tax	$ 15	$ 19	$ 47	$ 62