Pensions and Other Benefits - Amounts Recognized in Company's Consolidated Balance Sheets (Detail) - CAD CAD in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Pension asset	CAD 1,407	CAD 1,070
Pension and other benefit liabilities	(749)	(734)
Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Pension asset	1,407	1,070
Accounts payable and accrued liabilities	(10)	(10)
Pension and other benefit liabilities	(268)	(263)
Total amount recognized	1,129	797
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	(33)	(34)
Pension and other benefit liabilities	(481)	(471)
Total amount recognized	CAD (514)	CAD (505)