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Stock Repurchase Plan (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
Jan. 31, 2013
Jul. 31, 2012
Jan. 31, 2012
Jul. 16, 2008
Mar. 09, 2006
Payments for Repurchase of Equity [Abstract]          
Number of shares authorized to repurchase         3,000,000
Additional number of shares authorized to repurchase       3,000,000  
Number of shares repurchased since inception 4,949,111 4,949,111 4,468,181    
Value of stock repurchased since inception $ (193,192) $ (193,192) $ (170,696)    
Remaining shares available for repurchase under existing program 1,050,889