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Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)
 12 Months Ended 1 Months Ended 2 Months Ended 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2011
2010 ATM Program [Member]
Dec. 31, 2010
2010 ATM Program [Member]
Jan. 31, 2012
2011 ATM Program [Member]
Feb. 17, 2012
2011 ATM Program [Member]
Dec. 31, 2011
2011 ATM Program [Member]
Activity Under Share Offering Program [Line Items]                
Total net consideration $ 106,571,000 $ 231,651,000 $ 272,112,000 $ 13,847,000 $ 231,650,500 $ 3,200,000 $ 0 $ 92,723,600
Common shares sold       252,500 4,867,700 100,000 0 1,498,500
Average price per share       $ 55.81 $ 48.37 $ 62.41 $ 0.00 $ 62.98