Pensions and Other Benefits - Amounts Recognized in Accumulated Other Comprehensive Losses (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Amounts for defined benefit pension and other post-retirement plans not recognized in income	CAD (1,822)	CAD (1,504)
Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Other than deferred investment gains	2,842	3,144
Deferred investment gains	(366)	(1,101)
Prior service cost	(7)	(20)
Deferred income tax	(699)	(580)
Amounts for defined benefit pension and other post-retirement plans not recognized in income	1,770	1,443
Other Postretirement Benefit Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Other than deferred investment gains	66	77
Deferred investment gains	0	0
Prior service cost	3	4
Deferred income tax	(17)	(20)
Amounts for defined benefit pension and other post-retirement plans not recognized in income	CAD 52	CAD 61