Real Estate Investments (Schedule of Annualized Rental Income by Major Tenant) (Details) - Worldwide Plaza	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014
Cravath, Swaine & Moore, LLP
Revenue, Major Customer [Line Items]
Major tenant rental income, as a percentage of total annualized rental income	16.00%	18.00%
Nomura Holdings America, Inc.
Revenue, Major Customer [Line Items]
Major tenant rental income, as a percentage of total annualized rental income	10.00%	12.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 18

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

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-Section 50

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

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-Section 50

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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