Debt and Preferred Equity Investments (Details) (USD $)	12 Months Ended				12 Months Ended										1 Months Ended		1 Months Ended											1 Months Ended			1 Months Ended			1 Months Ended							1 Months Ended
	Dec. 31, 2011 sqft	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2011 Debt investment	Dec. 31, 2010 Debt investment	Dec. 31, 2011 Other loan with an initial maturity of September 2021	Dec. 31, 2010 Other loan with an initial maturity of September 2021	Dec. 31, 2011 Mortgage/Mezzanine loan with an initial maturity date of February 2016	Dec. 31, 2010 Mortgage/Mezzanine loan with an initial maturity date of February 2016	Dec. 31, 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	Dec. 31, 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. sqft	Dec. 31, 2010 Mezzanine Loan.	Dec. 31, 2011 Junior participation with an initial maturity of April 2008	Dec. 31, 2010 Junior participation with an initial maturity of April 2008	Dec. 31, 2011 Mortgage/Mezzanine loan with an initial maturity date of March 2017	Dec. 31, 2010 Mortgage/Mezzanine loan with an initial maturity date of March 2017	Dec. 31, 2011 Junior participation with an initial maturity date of November 2012	Dec. 31, 2010 Junior participation with an initial maturity date of November 2012	Dec. 31, 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 repaid in March 2011	Jan. 31, 2011 Junior participation sold in January 2011	Dec. 31, 2010 Junior participation sold in January 2011	Dec. 31, 2010 Junior participation with an initial maturity date of October 2011	Mar. 31, 2011 Mezzanine loan	Dec. 31, 2010 Mezzanine loan	Dec. 31, 2011 Mezzanine loan with an initial maturity date of July 2013	Nov. 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	Dec. 31, 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, 2011 Mortgage with an initial maturity date of February 2013	Sep. 30, 2011 Mortgage with an initial maturity date of February 2013	Dec. 31, 2010 Mortgage with an initial maturity date of February 2013	Dec. 31, 2011 Mezzanine loan with an initial maturity date of August 2012	Dec. 31, 2010 Mezzanine loan with an initial maturity date of August 2012	Dec. 31, 2010 Mezzanine loan with an initial maturity date of February 2014	Dec. 31, 2011 Mezzanine loan with an initial maturity date of May 2016	Dec. 31, 2011 Junior participation with an initial maturity date of June 2016	Dec. 31, 2011 Mezzanine loan with an initial maturity date of August 2014	Dec. 31, 2011 Mezzanine loan with an initial maturity date of July 2016	Dec. 31, 2011 Other Loan with an initial maturity of May 2012	Dec. 31, 2011 Mezzanine Loan with initial maturity date of October 2016	Dec. 31, 2011 Mezzanine Loan with initial maturity date of January 2015	Dec. 31, 2011 Mezzanine Loan with initial maturity date of July 2012
Debt and Preferred Equity Investments
Increase in debt and preferred equity investments (net of discounts), including investments classified as held-for-sale	$ 622,500,000	$ 520,700,000
Decrease in debt and preferred equity investments (net of discounts), including investments classified as held-for-sale	600,300,000	342,500,000
Debt investment
Aggregate weighted average current yield (as a percent)					7.50%	7.50%
Senior Financing					3,856,292,000		15,000,000		205,000,000		171,549,000		165,000,000								1,109,000,000		53,000,000		61,250,000											75,000,000						28,500,000			796,693,000			177,000,000	133,000,000	170,000,000	55,000,000	48,300,000	81,000,000	45,000,000	467,000,000
Amount Outstanding, Net of Discounts							3,500,000	3,500,000	64,973,000	60,407,000	46,416,000	46,358,000	40,375,000	39,711,000		27,187,000		15,697,000	8,725,000	9,938,000	108,817,000	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		202,136,000	7,650,000		15,000,000	86,339,000	86,339,000		3,000,000		26,000,000	8,392,000	13,536,000	38,892,000	17,112,000		60,000,000	35,000,000	3,196,000
Amount Outstanding, Net of Discounts	985,942,000	963,772,000			620,932,000	892,388,000																																											49,000,000				34,940,000	10,000,000	30,747,000
Loan Participation Amount included in Other assets & Other liabilities																																					7,400,000						28,500,000
Loan loss reserve	(50,175,000)	(61,361,000)	(93,844,000)	(98,916,000)	(19,125,000)	(40,461,000)
Realized additional income upon sale or disposal 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
Proceeds from sale of loan																												35,800,000
Ownership interest in consolidated joint venture holding the debt investment (as a percent)																																							88.00%
Interest in property (as a percent)																																		50.00%
Contribution of debt investment to a newly formed joint venture	286,571,000																																	286,600,000
Net proceeds from disposition of real estate/joint venture interest	160,548,000	623,121,000	27,946,000																															111,300,000
Mortgage loan assumed by joint venture	30,000,000																																	30,000,000
Interest rate on mortgage loan assumed by joint venture (as a percent)																																		1.16%
Additional funding provided																																									5,500,000
Area of property (in square feet)	31,426,318																205,000
Value of mortgage																																					15,000,000						31,500,000
Additional commitments to fund in connection with loan																																																					$ 15,000,000