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Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Total net consideration     $ 40,182,000 $ 604,459,000
2010 and 2011 ATM Programs [Member]
        
Total net consideration   83,836,300   128,128,000
Common shares sold   1,267,100   1,971,400
Average price per common share sold   $ 67.19   $ 66.01
2012 ATM Program [Member]
        
Total net consideration 30,816,200 84,711,800 40,181,700 84,711,800
Common shares sold 419,300 1,305,500 555,100 1,305,500
Average price per common share sold $ 74.74 $ 65.93 $ 73.73 $ 65.93
Maximum aggregate offering price of remaining common shares available for sale $ 82,700,000   $ 82,700,000