Pensions and Other Benefits - Amounts Recognized in Accumulated Other Comprehensive Losses (Detail) - CAD CAD in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Amounts recognized in accumulated other comprehensive loss	CAD 1,761	CAD 1,822
Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Other than deferred investment gains	2,555	2,842
Deferred investment gains	(178)	(366)
Prior service cost	(2)	(7)
Deferred income tax	(676)	(699)
Amounts recognized in accumulated other comprehensive loss	1,699	1,770
Other Postretirement Benefit Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Other than deferred investment gains	81	66
Deferred investment gains	0	0
Prior service cost	2	3
Deferred income tax	(21)	(17)
Amounts recognized in accumulated other comprehensive loss	CAD 62	CAD 52