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Share Repurchase Plan (Details) (USD $)
1 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2011
Apr. 30, 2011
Dec. 31, 2010
Aug. 31, 2012
Aug. 31, 2012
Nov. 30, 2011
Aug. 31, 2011
Aug. 31, 2010
Share Repurchase Plan (Textual) [Abstract]                
Share buy-back plan, amount authorized $ 50,000,000 $ 60,000,000 $ 25,000,000          
Share buy-back plan, additional amount authorized   35,000,000            
Share buy-back plan, expiration date   Apr. 04, 2013            
Share buy-back plan, number of shares repurchased       462,901   1,484,912    
Total cost of repurchased shares       $ 21,200,000 $ 39,840,000 $ 60,000,000 $ 41,399,000 $ 0