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Common Shares (Activity Under At-The-Market Share Offering Program) (Details) (USD $)
 6 Months Ended 3 Months Ended 6 Months Ended 1 Months Ended 3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
2010 and 2011 ATM Programs [Member]
Jun. 30, 2011
2010 and 2011 ATM Programs [Member]
Jun. 30, 2012
2010 and 2011 ATM Programs [Member]
Jun. 30, 2011
2010 and 2011 ATM Programs [Member]
Jul. 31, 2012
2012 ATM Program [Member]
Jun. 30, 2012
2012 ATM Program [Member]
Jun. 30, 2012
2012 ATM Program [Member]
Total net consideration $ 604,459,000 $ 37,126,000 $ 83,836,300 $ 33,331,600 $ 128,128,000 $ 37,126,100 $ 64,100,000 $ 84,711,800 $ 84,711,800
Common shares sold     1,267,100 550,400 1,971,400 621,700 900,000 1,305,500 1,305,500
Average price per common share sold     $ 67.19 $ 61.88 $ 66.01 $ 60.98 $ 69.34 $ 65.93 $ 65.93