Mortgage and Other Indebtedness (Details) (USD $) In Thousands, unless otherwise specified				6 Months Ended							3 Months Ended	6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011		Jun. 30, 2012 Secured Line of Credit 1 [Member]		Jun. 30, 2012 Secured Line of Credit 2 [Member]	Jun. 30, 2012 Secured Line of Credit 3 [Member]	Jun. 30, 2012 Unsecured lines of credit [Member]	Jun. 30, 2012 Unsecured Line of Credit 2 [Member]	Jun. 30, 2012 Letter of Credit [Member]	Jun. 30, 2012 Secured Lines of Credit [Member]	Jun. 30, 2012 Secured Lines of Credit [Member]	Jun. 30, 2012 Unsecured Term Loans[Member]	Jun. 30, 2012 Fixed-Rate Debt [Member]	Dec. 31, 2011 Fixed-Rate Debt [Member]		Jun. 30, 2012 Fixed-Rate Debt [Member] Non-recourse Loans on Operating Properties [Member]		Dec. 31, 2011 Fixed-Rate Debt [Member] Non-recourse Loans on Operating Properties [Member]		Jun. 30, 2012 Fixed-Rate Debt [Member] Recourse Term Loans on Operating Properties [Member]		Dec. 31, 2011 Fixed-Rate Debt [Member] Recourse Term Loans on Operating Properties [Member]		Jun. 30, 2012 Variable-Rate Debt [Member]	Dec. 31, 2011 Variable-Rate Debt [Member]		Jun. 30, 2012 Variable-Rate Debt [Member] Construction Loans [Member]	Dec. 31, 2011 Variable-Rate Debt [Member] Construction Loans [Member]		Jun. 30, 2012 Variable-Rate Debt [Member] Secured Lines of Credit [Member]	Dec. 31, 2011 Variable-Rate Debt [Member] Secured Lines of Credit [Member]		Jun. 30, 2012 Variable-Rate Debt [Member] Unsecured Term Loans[Member]	Dec. 31, 2011 Variable-Rate Debt [Member] Unsecured Term Loans[Member]		Jun. 30, 2012 Variable-Rate Debt [Member] Recourse Term Loans on Operating Properties [Member]		Dec. 31, 2011 Variable-Rate Debt [Member] Recourse Term Loans on Operating Properties [Member]		Jun. 30, 2012 Variable-Rate Debt [Member] Non Recourse Term Loans On Operating Properties [Member]		Dec. 31, 2011 Variable-Rate Debt [Member] Non Recourse Term Loans On Operating Properties [Member]		Jun. 30, 2012 Interest Rate Swap [Member]
Debt Instrument [Line Items]
Debt Instrument, Number of Instruments											3	3	2																																4
Line of Credit Facility, Maximum Borrowing Capacity				$ 525,000	[1]	$ 105,000	$ 520,000		$ 228,000	$ 50,000																					$ 1,150,000
Line of Credit Facility, Amount Outstanding	167,209			0	[1]	0	110,000	167,209	228,000
Total Outstanding														3,886,105	3,733,355		3,835,797	[2]	3,656,243	[2]	50,308		77,112		807,103	756,000		28,223	25,921		110,000	27,300		395,209	409,590		110,296		124,439		163,375		168,750
Weighted Average Interest Rate (in hundredths)	4.89%	4.99%	[3]					1.35%	1.84%		2.75%	2.75%		5.43%	5.54%	[3]	5.42%	[3]	5.55%	[3]	5.83%	[3]	5.89%	[3]	2.32%	2.18%	[3]	3.28%	3.25%	[3]		3.03%	[3]	1.88%	1.67%	[3]	2.39%	[3]	2.29%	[3]	3.47%	[3]	3.03%	[3]
Mortgage and other indebtedness	4,693,208	4,489,355
Debt Instrument, Maturity Date				Feb. 26, 2014		Jun. 01, 2015	Apr. 28, 2014	Nov. 30, 2012	Apr. 22, 2013
Interest Rate Terms						LIBOR plus a margin ranging from 1.75% to 2.75%		LIBOR plus a margin ranging from 0.95% to 1.40%	LIBOR plus a margin ranging from 1.50% to 1.80%			LIBOR plus an applicable spread, ranging from 2.00% to 3.00%
Line of credit, extended maturity date				Feb. 26, 2015		Jun. 01, 2016
Fees on amount of unused availability of secured lines of credit, maximum (in hundredths)											0.35%
Fees on amount of unused availability of secured lines of credit, minimum (in hundredths)											0.15%
Letters of Credit Outstanding, Amount				351	[1]					2,150
Additional secured and unsecured lines of credit with commitment										15,906
Covenants and Restrictions [Abstract]
Minimum loan amount in default payment for recourse loan	50,000
Minimum loan amount in default payment for non-recourse loan	$ 100,000

[1] There was an additional $351 outstanding on this secured line of credit as of June 30, 2012 forletters of credit. Up to $50,000 of the capacity on this line can be used for letters of credit.

[2] The Company has four interest rate swaps on notional amounts totaling $115,800 as of June 30, 2012 and $117,700 as of December 31, 2011 related to four variable-rate loans on operating properties to effectively fix the interest rate on the respective loans. Therefore, these amounts are reflected in fixed-rate debt at June 30, 2012 and December 31, 2011.

[3] Weighted-average interest rate includes the effect of debt premiums (discounts), but excludes amortization of deferred financing costs.