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Common Shares (Narrative) (Details) (USD $)
 1 Months Ended 6 Months Ended 1 Months Ended 3 Months Ended 6 Months Ended 1 Months Ended 3 Months Ended 6 Months Ended 1 Months Ended
Jan. 31, 2012
Jun. 30, 2012
Jun. 30, 2011
May 11, 2012
May 10, 2012
Dec. 31, 2011
May 31, 2011
2010 and 2011 ATM Programs [Member]
Mar. 31, 2010
2010 and 2011 ATM Programs [Member]
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]
May 31, 2012
2012 ATM Program [Member]
Jun. 30, 2012
2012 ATM Program [Member]
Jun. 30, 2012
2012 ATM Program [Member]
Aug. 03, 2012
2012 ATM Program [Member]
Jan. 31, 2012
January 2012 Public Equity Offering [Member]
Jun. 30, 2012
Line of Credit [Member]
Maximum aggregate offering price of common shares             $ 300,000,000 $ 250,000,000           $ 300,000,000          
Amount of current borrowing capacity under unsecured credit facility                                     500,000,000
Maximum aggregate offering price of remaining common shares available for sale                             213,900,000 213,900,000 148,900,000    
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   $ 391,600,000  
Common shares sold                 1,267,100 550,400 1,971,400 621,700 900,000   1,305,500 1,305,500   6,612,500  
Average price per common share sold                 $ 67.19 $ 61.88 $ 66.01 $ 60.98 $ 69.34   $ 65.93 $ 65.93      
Percentage of voting interests acquired 80.00%                                    
Number of unconsolidated equity method investments acquired 12                                    
Number of apartment units in wholly owned property acquired 4,034                                    
Number of common and preferred stock authorized to issue       185,000,000 110,000,000                            
Common shares, authorized   175,000,000   175,000,000   100,000,000                          
Preferred shares, authorized       10,000,000