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, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before income tax	$ 573	$ 470
Income tax recovery	(139)	(122)
Net of income tax	434	348
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	0	(1)
Recognition of net actuarial loss	21	30
Total before income tax	21	29
Income tax recovery	(5)	(7)
Net of income tax	$ 16	$ 22