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Investments in Unconsolidated Joint Ventures Investments in Unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands	Jul. 01, 2016 USD ($) ft² a Vehicles Buildings	Apr. 11, 2016 USD ($) ft² yr	Apr. 04, 2016 USD ($) ft² yr	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Equity Method Investments [Line Items]
Secured Debt				$ 2,077,707	$ 3,435,242
Construction Loans [Member] | Annapolis Junction Building Seven [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage			50.00%
Construction Loan			$ 21,500
Construction Loan Facility Borrowing Capacity			$ 22,000
Debt Instrument, Basis Spread on Variable Rate			1.65%
Number of extensions			1
Extension Option (in years) | yr			1
Net Rentable Area (in sf) | ft²			127,000
Secured Debt [Member] | Annapolis Junction Buidling One [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		50.00%
Secured Debt		$ 39,700
Debt Instrument, Basis Spread on Variable Rate		1.75%
Number of extensions		1
Extension Option (in years) | yr		3
Net Rentable Area (in sf) | ft²		118,000
Colorado Center [Member] | Colorado Center [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	49.80%
Net Rentable Area (in sf) | ft²	1,184,000
Aggregate purchase price	$ 511,100
Purchase price, net	$ 503,600
Number of buildings | Buildings	6
Land acquired (in acres) | a	15
Vehicles In Structured Parking | Vehicles	3,100

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Land acquired (in acres)

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number of buildings

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Vehicles In Structured Parking

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This element represents the carrying value of a short-term real estate loan to finance building costs. The funds are disbursed as needed or in accordance with a prearranged plan; generally, a portion of the funds is disbursed at inception and the remainder as construction progresses. The money is repaid on completion of the project, usually from the proceeds of a mortgage loan. The rate is normally higher than the prime rate, and there is usually an origination fee. The effective yield on these loans tends to be high, and the lender has a security interest in the real property. Note that there are separate concepts for the current and noncurrent portions of long-term construction loans.

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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 210.5-02.19(a)(1))
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Number 210
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-Paragraph 19, 20
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Number 210
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