Stock-Based Compensation - Narrative (Detail) CAD / shares in Units, CAD in Millions	12 Months Ended
	Dec. 31, 2015 CAD day CAD / shares shares	Dec. 31, 2014 CAD shares	Dec. 31, 2013 CAD shares
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense	CAD 66	CAD 110	CAD 92
Company's closing stock price (in CAD per share) | CAD / shares	CAD 176.73
Fair value of options at grant date	CAD 18	21	20
Total fair value of shares vested for stock option plan	CAD 17	15	5
Matching no. of DSU's that may be granted to key employees to meet share ownership target when acquiring Common Shares and DSU's acquired	25.00%
Ownership target threshold	5 years
Regular and Performance Stock Options [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense	CAD 15	18	17
Total unrecognized compensation related to share based compensation plans	CAD 13
Expected to be recognized over a weighted-average period	10 months 24 days
Performance Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense	CAD 55	CAD 50	25
Total unrecognized compensation related to share based compensation plans	CAD 17
Expected to be recognized over a weighted-average period	1 year 4 months 24 days
Units issued (shares) | shares	137,958
Grant date fair value	CAD 30
Other than options vesting period	3 years
PSU payout threshold	200.00%
PSU meeting threshold requirements | shares	300,095
PSU payout	CAD 79
Number of PSUs\RSUs exercised | shares	217,179	0
Number of trading days | day	30
Deferred Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense	CAD 10	CAD 28	32
Total unrecognized compensation related to share based compensation plans	CAD 1
Expected to be recognized over a weighted-average period	3 months 18 days
Other than options vesting period	48 months
Company granted DSUs | shares	21,690
Grant date fair value	CAD 5
Restricted Share Unit [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense	2	9	10
Total unrecognized compensation related to share based compensation plans	CAD 2
Units issued (shares) | shares	2,614
Grant date fair value	CAD 1
Restricted Share Unit [Member] | Maximum [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Other than options vesting period	36 months
Restricted Share Unit [Member] | Maximum [Member] | Cash Settled Restricted Stock Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Expected to be recognized over a weighted-average period	1 year 8 months 18 days
Employee Share Purchase Plan [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense	CAD 4	CAD 5	CAD 5
Period in which company's contribution are expensed	1 year
Employer contribution to employee share purchase plan	$1 for every $3 contributed by employees
The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan	6.00%
Ratio employer matches per dollar contributed	0.3333
Total number of shares purchased on behalf of participants | shares	131,703	176,906	271,934
Employer contributions	CAD 5	CAD 5	CAD 5