Pensions and Other Benefits - Amounts Recognized in Company's Consolidated Balance Sheets (Detail) - CAD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Pension asset	$ 1,243	$ 1,407
Pension and other benefit liabilities	(718)	(749)
Pension Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Pension asset	1,243	1,407
Accounts payable and accrued liabilities	(11)	(10)
Pension and other benefit liabilities	(255)	(268)
Total amount recognized	977	1,129
Other Post-retirement Benefit Plans, Defined Benefit [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Pension asset	0	0
Accounts payable and accrued liabilities	(34)	(33)
Pension and other benefit liabilities	(463)	(481)
Total amount recognized	$ (497)	$ (514)