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Summary of Restricted Stock Award and Restricted Stock Unit Activity (Detail) (Restricted Stock Awards and Restricted Stock Units, USD $)
3 Months Ended
Apr. 30, 2011
Restricted Stock Awards and Restricted Stock Units
 
Unreleased restricted stock  
Unreleased restricted stock at January 31, 2011 1,426,700
Awarded 675,000
Released (12,700)
Forfeited (33,300)
Unreleased restricted stock at April 30, 2011 2,055,700
Weighted average grant date fair value  
Unreleased restricted stock at January 31, 2011 $ 30.43
Awarded $ 43.15
Released $ 25.68
Forfeited $ 29.63
Unreleased restricted stock at April 30, 2011 $ 34.65