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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognition of net actuarial loss	$ 52	$ 45	$ 105	$ 90
Income tax recovery	(13)	(12)	(27)	(24)
Total net of income tax	$ 39	$ 33	$ 78	$ 66