Debt of the Operating Partnership (Debt Maturities Table) (Details) (USD $)			6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Revolving Credit Facility [Member]		Jun. 30, 2011 Unsecured Senior Notes [Member]	Jun. 30, 2011 Senior Notes [Member]	Dec. 31, 2010 Senior Notes [Member]	Jun. 30, 2011 Exchangeable Senior Debentures [Member]		Dec. 31, 2010 Exchangeable Senior Debentures [Member]	Jun. 30, 2011 Mortgage Loans [Member]		Dec. 31, 2010 Mortgage Loans [Member]	Jun. 30, 2011 Other Secured Loans [Member]	Dec. 31, 2010 Other Secured Loans [Member]
Remainder of 2011	$ 422,325,000		$ 341,417,000	[1]	$ 25,000,000			$ 48,301,000	[2]		$ 7,607,000	[3]
2012	164,743,000										164,743,000	[3]
2013	204,083,000				33,000,000						160,583,000	[3]		10,500,000
2014	487,396,000							266,400,000	[4]		220,996,000	[3]
2015	517,436,000				67,000,000	375,000,000					75,436,000	[3]
Thereafter	1,280,098,000				75,000,000	900,000,000					305,098,000	[3]
Subtotal	3,076,081,000		341,417,000	[1]	200,000,000	1,275,000,000		314,701,000			934,463,000	[3]		10,500,000
Unamortized discounts	(9,526,000)					(9,413,000)	(8,970,000)	(113,000)		(1,456,000)
Unamortized premium	1,022,000										1,022,000	[3]	1,101,000
Revolving credit facility	341,417,000	333,534,000	341,417,000	[1]
Unsecured Senior Note	3,076,081,000		341,417,000	[1]	200,000,000	1,275,000,000		314,701,000			934,463,000	[3]		10,500,000
Senior Notes						1,265,587,000	866,030,000
Exchangeable Senior Debenture	314,588,000	353,702,000						314,588,000		353,702,000
Mortgage Loans	935,485,000	1,043,188,000									935,485,000	[3]	1,043,188,000
Other secured loan	10,500,000	10,500,000												10,500,000	10,500,000
Total indebtedness	3,067,577,000	2,806,954,000
Revolving credit facility commitments extension (years)			1
Limited recourse guarantees											60,100,000
Partial letter of credit											$ 47,300,000

[1] Subject to a one-year extension option exercisable by us. The bank group is obligated to grant the extension option provided we give proper notice, we make certain representations and warranties and no default exists under the revolving credit facility.

[2] Assumes maturity of the 2026 Debentures at first redemption date in August 2011.

[3] Our mortgage loans are generally non-recourse to us, subject to carve outs for specified actions by us or specified undisclosed environmental liabilities. As of June 30, 2011, we had provided limited recourse guarantees with respect to approximately $60.1 million principal amount of the outstanding mortgage indebtedness, and partial letter of credit support with respect to approximately an additional $47.3 million of the outstanding mortgage indebtedness.

[4] Assumes maturity of the 2029 Debentures at first redemption date in April 2014.