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Common Shares (Narrative) (Details) (USD $)
 1 Months Ended 3 Months Ended 6 Months Ended
May 31, 2011
Mar. 31, 2010
Sep. 30, 2011
Jun. 30, 2011
Jun. 30, 2010
Dec. 31, 2010
Common Shares            
Number of common and preferred stock authorized to issue       110,000,000    
Common shares, authorized       100,000,000   100,000,000
Preferred shares authorized       10,000,000    
Common shares outstanding       70,700,000    
Preferred shares outstanding       0    
Maximum aggregate offering price of common shares $ 300,000,000 $ 250,000,000        
Common shares sold     500,000      
Average price per common share issued     $ 65.77      
Total net consideration     32,400,000 37,126,000 106,422,000  
Maximum aggregate offering price of remaining common shares available for sale     $ 243,300,000