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Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2012
2010 and 2011 ATM Programs [Member]
Dec. 31, 2011
2010 and 2011 ATM Programs [Member]
Dec. 31, 2013
2012 ATM Program [Member]
Dec. 31, 2012
2012 ATM Program [Member]
Feb. 21, 2014
2012 ATM Program [Member]
Maximum aggregate offering price of remaining common shares available for sale               $ 82,700,000
Total net consideration $ 40,044,000 $ 693,355,000 $ 106,571,000 $ 128,128,000 $ 106,570,600 $ 40,044,100 $ 173,607,500  
Common shares sold 555 11,192 1,751 1,971,400 1,751,000 555,100 2,607,900  
Average price per common share sold       $ 66.01 $ 61.95 $ 73.73 $ 67.63