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Net Income per Common Share - Additional Information (Detail) (USD $)
3 Months Ended 6 Months Ended
Jul. 31, 2014
Jul. 31, 2013
Jul. 31, 2014
Jul. 31, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive common shares 1,183,390 100,000 1,065,670 50,000
Anti-dilutive common shares exercise price, minimum $ 35.12   $ 35.12  
Anti-dilutive common shares exercise price, maximum $ 46.02   $ 46.02  
Anti-dilutive common shares exercise price   $ 46.02   $ 46.02
Performance Stock Units
       
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive common shares     1,744,034 377,100