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Shareholders' capital - Fair Value of Share Options Granted (Detail) - CAD / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Equity [Abstract]    
Risk-free interest rate 1.30% 2.00%
Expected volatility 38.00% 38.00%
Expected dividend yield 4.00% 3.80%
Expected life 8 years 8 years
Weighted average grant date fair value per option (CAD per share) CAD 2.45 CAD 2.00