Debt of the Operating Partnership (4.125 Notes) (Narrative) (Details) (USD $)	3 Months Ended			6 Months Ended			12 Months Ended	48 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Jun. 30, 2011		Jun. 30, 2010	Dec. 31, 2010	Dec. 31, 2010	Aug. 15, 2021	Aug. 16, 2016	Aug. 15, 2011	Aug. 15, 2006
Unsecured Senior Note	$ 3,076,081,000			$ 3,076,081,000
Loss from early extinguishment of debt	(363,000)		(1,541,000)	(978,000)		(1,541,000)
Exchangeable Senior Debentures [Member]
Unsecured Senior Note	314,701,000			314,701,000
Exchangeable Senior Debentures [Member] | 4.125% Exchangeable Senior Debentures due 2026 [Member]
Interest Rate at June 30, 2011	4.125%			4.125%
Unsecured Senior Note	48,300,000			48,300,000
Maturity Date				Aug. 15, 2026	[1]
Right to redeem date	August 18, 2011			August 18, 2011
4.125% Exchangeable Senior Debentures due 2026 [Member]
Notes issuance date				August 15, 2006
Exchangeable senior debenture face amount at time of issuance												172,500,000
Unsecured Senior Note	48,301,000			48,301,000			88,758,000	88,758,000
Debt issuance costs				5,400,000
Interest payable dates	August 15 and February 15			August 15 and February 15
First required interest payment				February 15, 2007
Common stock exchange rate (% of shares per $1,000 in principal)							0.03182%	0.03068%
Reference dividend rate		$ 0.265
Dates which the holders of may require the Operating Partnership to repurchase the debentures									August 15, 2021	August 15, 2016	August 15, 2011
Purchase price/Redemption price percentage of debenture principal							100
Exchange price	$ 30.99			$ 30.99			$ 31.42	$ 31.42
Principal amount of debentures exchanged				40,500,000
Privately issued common stock issued in exchange				372,068
Cash issued in exchange for debentures				52,200,000
Loss from early extinguishment of debt	$ 100,000		$ 1,500,000	$ 700,000		$ 1,500,000

[1] The holders of the debentures have the right to require the Operating Partnership to repurchase the debentures in cash in whole or in part for a price of 100% of the principal amount plus accrued and unpaid interest on each of August 15, 2011, August 15, 2016 and August 15, 2021. We have the right to redeem the debentures in cash for a price of 100% of the principal amount plus accrued and unpaid interest commencing on August 18, 2011.