Pensions and Other Benefits - Amounts Recognized in Company's Consolidated Balance Sheets (Detail) - CAD CAD in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Pension asset	CAD 1,070	CAD 1,401
Pension and other benefit liabilities	(734)	(758)
Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Pension asset	1,070	1,401
Accounts payable and accrued liabilities	(10)	(10)
Pension and other benefit liabilities	(263)	(285)
Total amount recognized	797	1,106
Other Postretirement Benefit Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Pension asset	0	0
Accounts payable and accrued liabilities	(34)	(34)
Pension and other benefit liabilities	(471)	(473)
Total amount recognized	CAD (505)	CAD (507)