Debt of the Operating Partnership (Exchangeable Senior Debentures) (Details) (USD $) In Thousands, unless otherwise specified					6 Months Ended			6 Months Ended			12 Months Ended					12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Exchangeable Senior Debentures [Member]	Dec. 31, 2010 Exchangeable Senior Debentures [Member]	Jun. 30, 2011 Exchangeable Senior Debentures [Member] 4.125% Exchangeable Senior Debentures due 2026 [Member]		Dec. 31, 2010 Exchangeable Senior Debentures [Member] 4.125% Exchangeable Senior Debentures due 2026 [Member]	Jun. 30, 2011 Exchangeable Senior Debentures [Member] 5.50% Exchangeable Senior Debentures due 2029 [Member]		Dec. 31, 2010 Exchangeable Senior Debentures [Member] 5.50% Exchangeable Senior Debentures due 2029 [Member]	Dec. 31, 2010 4.125% Exchangeable Senior Debentures due 2026 [Member]	Aug. 15, 2021 4.125% Exchangeable Senior Debentures due 2026 [Member]	Aug. 16, 2016 4.125% Exchangeable Senior Debentures due 2026 [Member]	Aug. 15, 2011 4.125% Exchangeable Senior Debentures due 2026 [Member]	Jun. 30, 2011 4.125% Exchangeable Senior Debentures due 2026 [Member]	Dec. 31, 2010 5.50% Exchangeable Senior Debentures due 2029 [Member]	Apr. 15, 2024 5.50% Exchangeable Senior Debentures due 2029 [Member]	Apr. 15, 2019 5.50% Exchangeable Senior Debentures due 2029 [Member]	Apr. 15, 2014 5.50% Exchangeable Senior Debentures due 2029 [Member]	Jun. 30, 2011 5.50% Exchangeable Senior Debentures due 2029 [Member]
Interest Rate at June 30, 2011					4.125%			5.50%												5.50%
Maturity Date					Aug. 15, 2026	[1]		Apr. 15, 2029	[2]
Unamortized discounts	$ (9,526)		$ (113)	$ (1,456)							$ (1,456)				$ (113)
Exchangeable senior debentures, net of discount	$ 314,588	$ 353,702	$ 314,588	$ 353,702	$ 48,301		$ 88,758	$ 266,400		$ 266,400
Purchase price/Redemption price percentage of debenture principal											100					100
Dates which the holders of may require the Operating Partnership to repurchase the debentures												August 15, 2021	August 15, 2016	August 15, 2011			April 15, 2024	April 15, 2019	April 15, 2014
Earliest date which The Company may repurchase debentures					August 18, 2011			April 18, 2014

[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.

[2] 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 April 15, 2014, April 15, 2019 and April 15, 2024. 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 April 18, 2014.