Pensions and Other Benefits - Amounts Recognized in Accumulated Other Comprehensive Losses (Detail) - CAD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Amounts recognized in accumulated other comprehensive loss	$ 2,094	$ 1,761
Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Other than deferred investment gains	2,233	2,555
Deferred investment gains	611	(178)
Prior service cost	0	(2)
Deferred income tax	(797)	(676)
Amounts recognized in accumulated other comprehensive loss	2,047	1,699
Other Post-retirement Benefit Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Other than deferred investment gains	61	81
Deferred investment gains	0	0
Prior service cost	2	2
Deferred income tax	(16)	(21)
Amounts recognized in accumulated other comprehensive loss	$ 47	$ 62