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Common Shares (Tables)
 12 Months Ended
Dec. 31, 2011
2010 ATM Program [Member]
  
Activity Under Share Offering Program [Line Items]  
Activity Under At-The-Market Share Offering Program
September 30, September 30,
       Year Ended December 31,  
     2011        2010  

Total net consideration

      $ 13,847.0         $ 231,650.5   

Common shares sold

        252.5           4,867.7   

Average price per share

      $ 55.81         $ 48.37   
2011 ATM Program [Member]
  
Activity Under Share Offering Program [Line Items]  
Activity Under At-The-Market Share Offering Program
September 30,
       Year Ended
December 31, 2011		  

Total net consideration

      $ 92,723.6   

Common shares sold

        1,498.5   

Average price per share

      $ 62.98