Fair Value Measurements (Details) (USD $)	12 Months Ended					12 Months Ended																	3 Months Ended	9 Months Ended	12 Months Ended				9 Months Ended	12 Months Ended				9 Months Ended	12 Months Ended				9 Months Ended
	Dec. 31, 2011	Sep. 30, 2012 Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2011 Fair Value, Inputs, Level 2 [Member]	Sep. 30, 2012 Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2011 Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member]	Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]		Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member] Fair Value, Inputs, Level 1 [Member]		Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member] Fair Value, Inputs, Level 2 [Member]		Dec. 31, 2011 Fair Value, Measurements, Nonrecurring [Member] Fair Value, Inputs, Level 3 [Member]		Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value, Fair Value Disclosure [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]	Sep. 30, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]	Sep. 30, 2012 Blackstone Portfolio [Member]	Sep. 30, 2012 Blackstone Portfolio [Member]	Dec. 31, 2011 Operating and development properties Fair Value, Measurements, Nonrecurring [Member]		Dec. 31, 2011 Investment in real estate partnerships Fair Value, Measurements, Nonrecurring [Member]		Sep. 30, 2012 Minimum Notes Receivable [Member]	Dec. 31, 2011 Minimum Yield rates	Dec. 31, 2011 Minimum Rental growth rates	Dec. 31, 2011 Minimum Discount rates	Dec. 31, 2011 Minimum Terminal cap rates	Sep. 30, 2012 Maximum Notes Receivable [Member]	Dec. 31, 2011 Maximum Yield rates	Dec. 31, 2011 Maximum Rental growth rates	Dec. 31, 2011 Maximum Discount rates	Dec. 31, 2011 Maximum Terminal cap rates	Sep. 30, 2012 Line of Credit Facility - Unsecured [Member]	Sep. 30, 2012 Notes Payable, Other Payables [Member] Minimum	Sep. 30, 2012 Notes Payable, Other Payables [Member] Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Ratios Related To Real Estate Fair Value Inputs																														7.50%	2.00%	8.50%	8.00%		9.00%	3.00%	10.00%	9.50%
Property, Plant, and Equipment, Fair Value Disclosure							$ 5,520,000	[1]	$ 0	[1]	$ 0	[1]	$ 5,520,000	[1]
Proceeds from Sale of Real Estate																							321,000,000
Disposal Date																							July 25, 2012
Equity Method Investment, Other than Temporary Impairment																											(4,580,000)	[1]
Fair Value, Option, Changes in Fair Value, Gain (Loss)						(16,423,000)																			(11,843,000)	[1]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures							1,893,000	[1]	0	[1]	0	[1]	1,893,000	[1]
Impairment of Real Estate	16,400,000																						1,100,000	19,100,000
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value, Discount Rate																													7.04%					8.10%					1.60%	2.30%	3.50%
Notes Receivable, Fair Value				23,700,000	35,300,000
Trading Securities, Fair Value Disclosure															23,208,000	21,713,000	23,208,000	21,713,000	0	0	0	0
Notes Payable, Fair Value		2,010,000,000	2,090,000,000
Total															23,208,000	21,713,000	23,208,000	21,713,000	0	0	0	0
Derivative Liabilities															(86,000)	(37,000)	0	0	(88,000)	(38,000)	2,000	1,000
Assets, Fair Value Disclosure, Nonrecurring							7,413,000		0		0		7,413,000
Debt Instrument, Fair Value Disclosure		$ 64,800,000	$ 40,000,000

[1] Excludes impairments for properties sold during the year ended December 31, 2011.