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
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income tax recovery	$ (122)	$ (148)
Net of income tax	348	431
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	(1)	(1)
Recognition of net actuarial loss	30	39
Total before income tax	29	38
Income tax recovery	(7)	(10)
Net of income tax	$ 22	$ 28