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Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Total net consideration     $ 693,375,000 $ 69,863,000
2010 and 2011 ATM Programs [Member]
        
Total net consideration 0 32,736,800 128,128,000 69,862,900
Common shares sold 0 506,200 1,971,400 1,127,900
Average price per common share sold $ 0 $ 65.76 $ 66.01 $ 63.13
2012 ATM Program [Member]
        
Total net consideration $ 88,915,900   $ 173,627,600  
Common shares sold 1,302,500   2,607,900  
Average price per common share sold $ 69.34   $ 67.63