Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Amounts in Pension and Post-retirement Defined Benefit Plans Reclassified from AOCL (Details) - Reclassification out of Accumulated Other Comprehensive Income - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognition of net actuarial loss	$ 53	$ 45
Income tax recovery	(14)	(12)
Total net of income tax	$ 39	$ 33