Debt and Preferred Equity Investments (Details) (USD $)	6 Months Ended				6 Months Ended	12 Months Ended									1 Months Ended		1 Months Ended												1 Months Ended			1 Months Ended				1 Months Ended						1 Months Ended
	Jun. 30, 2011 SquareFoot	Jun. 30, 2010	Dec. 31, 2010	Dec. 31, 2009	Jun. 30, 2011 Debt investment	Dec. 31, 2010 Debt investment	Jun. 30, 2011 Other loan with an initial maturity of September 2021	Dec. 31, 2010 Other loan with an initial maturity of September 2021	Jun. 30, 2011 Mezzanine loan with an initial maturity date of February 2016	Dec. 31, 2010 Mezzanine loan with an initial maturity date of February 2016	Jun. 30, 2011 Mortgage/Mezzanine loan with an initial maturity date of May 2016	Dec. 31, 2010 Mortgage/Mezzanine loan with an initial maturity date of May 2016	Jun. 30, 2011 Mezzanine loan with an initial maturity date of November 2016	Dec. 31, 2010 Mezzanine loan with an initial maturity date of November 2016	Feb. 28, 2011 Mezzanine loan sold in February 2011	Dec. 31, 2010 Mezzanine loan sold in February 2011	May 31, 2011 Mezzanine Loan. SquareFoot	May 30, 2011 Mezzanine Loan. SquareFoot	Dec. 31, 2010 Mezzanine Loan.	Jun. 30, 2011 Junior participation with an initial maturity of April 2008	Dec. 31, 2010 Junior participation with an initial maturity of April 2008	Jun. 30, 2011 Mezzanine loan with an initial maturity date of March 2017	Dec. 31, 2010 Mezzanine loan with an initial maturity date of March 2017	Jun. 30, 2011 Junior participation with an initial maturity date of November 2011	Dec. 31, 2010 Junior participation with an initial maturity date of November 2011	Jun. 30, 2011 Junior participation with an initial maturity date of June 2012	Dec. 31, 2010 Junior participation with an initial maturity date of June 2012	Dec. 31, 2010 Junior participation with an initial maturity date of December 2010	May 31, 2011 Junior participation sold in May 2011	Dec. 31, 2010 Junior participation sold in May 2011	Dec. 31, 2010 Mezzanine loan sold in March 2011	Jan. 31, 2011 Junior participation sold in January 2011	Dec. 31, 2010 Junior participation sold in January 2011	Jun. 30, 2011 Junior participation with an initial maturity date of October 2011	Dec. 31, 2010 Junior participation with an initial maturity date of October 2011	Mar. 31, 2011 Mezzanine loan	Dec. 31, 2010 Mezzanine loan	Jun. 30, 2011 Mezzanine loan with an initial maturity date of July 2013	Dec. 31, 2010 Mezzanine loan with an initial maturity date of July 2013	Jun. 30, 2011 Mortgage with an initial maturity date of June 2012	Dec. 31, 2010 Mortgage with an initial maturity date of June 2012	Jun. 30, 2011 Mortgage with an initial maturity date of February 2013	Dec. 31, 2010 Mortgage with an initial maturity date of February 2013	Jun. 30, 2011 Mezzanine loan with an initial maturity date of August 2011	Dec. 31, 2010 Mezzanine loan with an initial maturity date of August 2011	Jun. 30, 2011 Mezzanine loan with an initial maturity date of February 2014	Dec. 31, 2010 Mezzanine loan with an initial maturity date of February 2014	Jun. 30, 2011 Mezzanine loan with an initial maturity date of May 2016	Jun. 30, 2011 Junior participation with an initial maturity date of June 2016	Dec. 31, 2010 Loan loss reserve - debt investments
Debt and Preferred Equity Investments
Increase in debt and preferred equity investments (net of discounts), including investments classified as held-for-sale	$ 162,900,000	$ 181,200,000
Decrease in debt and preferred equity investments (net of discounts), including investments classified as held-for-sale	544,300,000	99,400,000
Debt investment
Aggregate weighted average current yield (as a percent)					5.99%	5.99%
Senior Financing					3,228,852,000		15,000,000		205,000,000		172,669,000		165,000,000									1,139,000,000		53,000,000		61,250,000								70,800,000				75,000,000						796,693,000		165,440,000		177,000,000	133,000,000
Principal Outstanding							3,500,000	3,500,000	63,711,000	60,407,000	46,380,000	46,358,000	40,282,000	39,711,000		27,187,000			15,697,000	9,938,000	9,938,000	83,378,000	84,062,000	11,000,000	11,000,000	10,875,000	10,875,000	5,866,000		47,484,000	137,222,000		42,439,000	9,200,000	9,200,000		202,136,000	15,000,000	15,000,000	86,339,000	86,339,000	31,500,000	26,000,000	13,536,000	13,536,000	38,682,000	38,892,000	17,668,000		(40,461,000)
Principal Outstanding	582,418,000		963,772,000		511,589,000	892,388,000																																											49,000,000
Amount Outstanding	41,800,000		61,361,000	93,844,000	(18,400,000)	(40,461,000)
Realized additional income upon the sale of loan															6,200,000														1,200,000			1,300,000				38,700,000
Pari passu interest in the loan of another participant																				12,200,000
Proceed from sale of loan																													35,800,000
Ownership interest in consolidated joint venture holding the debt investment (as a percent)																																				50.00%				88.00%
Ownership Interest (as a percent)																																				50.00%
Contribution of debt investment to a newly formed joint venture	286,571,000																																			286,600,000
Proceeds from newly formed joint venture																																				111,300,000
Mortgage loan assumed by joint venture	30,000,000																																			30,000,000
Interest rate on mortgage loan assumed by joint venture																																				1.16%
Additional funding provided																																										$ 5,500,000
Square-foot of property acquired (in square foot)	30,194,138																205,000	205,000