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Risk Management and Use of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended			6 Months Ended
	Mar. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Risk Management and use of Financial Instruments (Textuals) [Abstract]
Anticipation of the offering of unsecured guaranteed notes due	$ 325.0		$ 325.0
Interest rates of unsecured guaranteed notes due	4.95%		4.95%
Treasury lock agreements					seven intra-day treasury lock agreements
Total notional amount of the treasury lock agreements		230.0			230.0
Expiration period of treasury lock agreements					7 years
Treasury rates, Minimum			2.858%
Treasury rates, Average			2.873%
Treasury rates, Maximum			2.891%
Fair Value Treasury Lock Agreements	0.6
Total expense of debt offering		$ 0.6			$ 0.6
No. of tenants accounted for 10% or more of the Company's rents		0		0	0	0