Amounts Recorded in Consolidated Balance Sheet and Accumulated Other Comprehensive Income (Detail) - CAD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Schedule Of Defined Benefit Plans Disclosures [Line Items]
Current liabilities		$ (58)	$ (58)
Other long-term obligations	[1]	(2,105)	(1,822)
Pension benefits
Schedule Of Defined Benefit Plans Disclosures [Line Items]
Current liabilities		(27)	(27)
Other long-term obligations		(1,263)	(1,160)
Total recorded		(1,290)	(1,187)
Net actuarial loss (gain)		2,232	2,256
Prior service cost		269	283
Total recorded in accumulated other comprehensive income, before-tax		2,501	2,539
Other postretirement benefits
Schedule Of Defined Benefit Plans Disclosures [Line Items]
Current liabilities		(31)	(31)
Other long-term obligations		(842)	(662)
Total recorded		(873)	(693)
Net actuarial loss (gain)		272	133
Total recorded in accumulated other comprehensive income, before-tax		$ 272	$ 133

[1] Total recorded employee retirement benefits obligations also included $58 million in current liabilities (2019 – $58 million).