Debt of the Operating Partnership (5.50 Notes) (Narrative) (Details) (5.50% Exchangeable Senior Debentures due 2029 [Member], USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2011		Dec. 31, 2010	Apr. 15, 2024	Apr. 15, 2019	Apr. 15, 2014	Apr. 20, 2009
Notes issuance date	April 20, 2009
Exchangeable senior debenture face amount at time of issuance							$ 266,400,000
Interest Rate at June 30, 2011	5.50%
Debt issuance costs	$ 7,800,000
Interest payable dates	October 15 and April 15
First required interest payment	October 15, 2009
Common stock exchange rate (% of shares per $1,000 in principal)	0.02354%		0.02326%
Reference dividend rate	$ 0.33
Dates which the holders of may require the Operating Partnership to repurchase the debentures				April 15, 2024	April 15, 2019	April 15, 2014
Purchase price/Redemption price percentage of debenture principal			100
Exchangeable Senior Debentures [Member]
Interest Rate at June 30, 2011	5.50%
Maturity Date	Apr. 15, 2029	[1]
Right to redeem date	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 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.