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Investment in Unconsolidated Ventures (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 9 Months Ended |
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Sep. 30, 2012 A Properties Room sqft Real_Estate_Ventures Buildings | Dec. 31, 2011
Investment in Unconsolidated Ventures (Textuals) [Abstract] |  |
Company's share of equity (Company's basis) | $ 179,037 | $ 115,807
Number of unconsolidated Real Estate Ventures | 19 |
Number of real estate ventures that owns 51 office buildings | 15 |
Number of office buildings owned by 15 Real Estate Ventures | 54 |
Net rentable square feet of 51 office buildings owned the 15 Real Estate Ventures | 7,000,000 |
Number of real estate ventures that owns undeveloped land | 3 |
Area of undeveloped parcels of land (in acres) owned by a Real Estate Venture | 24 |
Number of real estate ventures developed hotel property | 1 |
Number of rooms in a hotel property developed by one Real Estate Venture | 137 |
Minimum ownership interest in unconsolidated Real Estate Ventures | 20.00% |
Maximum ownership interest in unconsolidated Real Estate Ventures | 65.00% |

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- Definition Aggregate approximate area of office buildings. + References No definition available. + Details
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- Definition Area owned by Real estate venture of undeveloped parcels of land (in acres). + References No definition available. + Details
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- Definition Number of office buildings in Real Estate Ventures. + References No definition available. + Details
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- Definition Number of rooms contain in one real estate venture developed as Hotel Property. + References No definition available. + Details
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- Definition This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=16385135&loc=d3e33749-111570 + Details
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