Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Amounts in Pension and Post-Retirement Defined Benefit Plans Reclassification from AOCL (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income tax expense (recovery)	$ 706	$ 637	$ 93
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	0	(2)
Recognition of net actuarial loss	84	117
Total before income tax	84	115
Income tax expense (recovery)	(20)	(31)
Total net of income tax	$ 64	$ 84