Common Shares (Narrative) (Details) (USD $)	1 Months Ended		12 Months Ended					1 Months Ended		12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended
	Dec. 31, 2012	Jan. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	May 11, 2012	May 10, 2012	May 31, 2011 2010 and 2011 ATM Programs [Member]	Mar. 31, 2010 2010 and 2011 ATM Programs [Member]	Dec. 31, 2012 2010 and 2011 ATM Programs [Member]	Dec. 31, 2011 2010 and 2011 ATM Programs [Member]	Dec. 31, 2010 2010 and 2011 ATM Programs [Member]	May 31, 2012 2012 ATM Program [Member]	Dec. 31, 2012 2012 ATM Program [Member]	Feb. 15, 2013 2012 ATM Program [Member]	Jan. 31, 2012 January 2012 Public Equity Offering [Member]	Dec. 31, 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
Total net consideration			693,355,000	106,571,000	231,651,000					128,128,000	106,570,600	231,650,500		173,607,500		391,600,000
Common shares sold			11,192	1,751	4,868					1,971,400	1,751,000	4,867,700		2,607,900		6,612,500
Average price per common share sold										$ 66.01	$ 61.95	$ 48.37		$ 67.63
Maximum aggregate offering price of remaining common shares available for sale															$ 123,600,000
Percentage of voting interests acquired	50.00%	80.00%	50.00%
Number of unconsolidated equity method investments acquired	1	12
Number of apartment units in wholly owned property acquired	320	4,034	2,114
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		175,000,000
Preferred shares, authorized						10,000,000
Common Stock, Shares, Outstanding	84,400,000		84,400,000