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Restricted Stock (Detail) (USD $)
12 Months Ended
Jan. 31, 2013
Jan. 31, 2012
Number of share    
Unvested as of January 31, 2011 925,900 658,882
Granted 358,985 437,850
Vested (226,341) (162,332)
Canceled (1,813) (8,500)
Unvested as of January 31, 2012 1,056,731 925,900
Weighted average grant date fair value    
Unvested as of January 31, 2011 $ 22.60 $ 13.86
Granted $ 35.18 $ 30.80
Vested $ 12.54 $ 9.16
Canceled $ 29.28 $ 24.03
Unvested as of January 31, 2012 $ 29.01 $ 22.60