Risk Management and Use of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended			9 Months Ended
	Mar. 31, 2011	Sep. 30, 2011 Property Tenants Year ac Room Buildings sqft	Mar. 31, 2011	Sep. 30, 2010 Property Tenants	Sep. 30, 2011 Property Tenants Year sqft ac Room Buildings	Sep. 30, 2010 Tenants Property
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