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Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2012
Two Thousand Eleven Atm Program [Member]
Mar. 31, 2013
2012 ATM Program [Member]
May 03, 2013
2012 ATM Program [Member]
Maximum aggregate offering price of remaining common shares available for sale         $ 114,000,000
Total net consideration $ 9,366,000 $ 435,911,000 $ 44,291,700 $ 9,365,500  
Common shares sold     704,200 135,700  
Average price per common share sold     $ 63.89 $ 70.63