Investments in Unconsolidated Entities (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended		3 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2010 Forest City Enterprises [Member] properties	Sep. 30, 2012 Forest City Enterprises [Member]	Jun. 30, 2012 Chartwell [Member] properties
Schedule of Equity Method Investments [Line Items]
Interest in seven-building life science campus			49.00%
Number of buildings acquired			7
Equity method investments	$ 227,735	$ 779	$ 176,579		$ 223,134
Unamortized investment in joint venture primarily attributable to real estate and related intangible assets				$ 1,791
Investment Percentage Range	10% to 50%
Number of properties in portfolio					42
Controlling interest in number of properties					3
Number of properties under partnership not owned by company					39
Percentage of interest owned by company in partnership					50.00%