Stock-Based Compensation - Narrative (Detail) - CAD CAD / shares in Units, CAD in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense		CAD 51	CAD 66	CAD 110
Company's closing stock price (in CAD per share)		CAD 191.56
Fair value of options at grant date		CAD 16	18	21
Total fair value of shares vested for stock option plan		CAD 15	17	15
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 14	15	18
Total unrecognized compensation related to share based compensation plans		CAD 8
Expected to be recognized over a weighted-average period		1 year 30 days
Performance Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense		CAD 29	CAD 55	50
Total unrecognized compensation related to share based compensation plans		CAD 15
Expected to be recognized over a weighted-average period		1 year 4 months 24 days
Units issued (shares)		147,157
Grant date fair value		CAD 25
Other than options vesting period		3 years
PSU payout threshold		118.00%	200.00%
PSU meeting threshold requirements		134,063	300,095
PSU payout	CAD 31	CAD 31	CAD 79
Number of awards exercised	83,466	83,466	217,179
Number of trading days		30 days	30 days
Deferred Share Units [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense		CAD 2	CAD (10)	28
Total unrecognized compensation related to share based compensation plans		CAD 1
Expected to be recognized over a weighted-average period		1 year 2 months 27 days
Other than options vesting period		48 months
Number of awards exercised		87,996	11,784
Company granted DSUs		31,069
Grant date fair value		CAD 5
Employee Share Purchase Plan [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-based compensation expense		CAD 5	CAD 4	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		140,560	131,703	176,906
Employer contributions		CAD 5	CAD 5	CAD 5