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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

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- Definition Interest rate stated in the contractual debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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