Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Loss - Amounts in Pension and Post-Retirement Defined Benefit Plans Reclassification from AOCL (Details) - Reclassification out of Accumulated Other Comprehensive Income [Member] - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	$ (1)	$ 0
Recognition of net actuarial loss	180	84
Total before income tax	179	84
Income tax recovery	(47)	(20)
Total net of income tax	$ 132	$ 64