Commitments and Contingencies - Supply Commitments (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Supply Commitment [Line Items]
Development in Process	$ 269,518,000us-gaap_DevelopmentInProcess	$ 277,411,000us-gaap_DevelopmentInProcess
Construction in Progress [Member]
Supply Commitment [Line Items]
Net Rentable Area	3,800,000
Number of Real Estate Properties	21
Total anticipated development cost	412,900,000
Deferred Costs, Leasing, Gross	8,000,000
Construction in Progress [Member] \| Comcast Innovation and Technology Center [Member]
Supply Commitment [Line Items]
Net Rentable Area	1,300,000
Total anticipated development cost	932,000,000
Comcast Innovative and Technology Center [Abstract]
Number of equity method investments	2
Number of stories in a high-rise building	59
Number of hotel rooms	222
Anticipated investment in joint ventures	185,000,000
Equity Method Investment, Ownership Percentage	20.00%
Development Not Yet Commenced [Member]
Supply Commitment [Line Items]
Total anticipated development cost	167,000,000
Buildings and land parcels [Member]
Supply Commitment [Line Items]
Remaining amount committed or obligated to pay for the benefit of current or future tenants	14,800,000
Tenant Improvements [Member]
Supply Commitment [Line Items]
Tenant improvements not yet completed	$ 41,500,000

Total anticipated cost of investment in assets including but not limited to assets under construction.

No definition available.

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- Definition

Anticipated balance in investment in unconsolidated joint ventures after development is completed.

+ References

No definition available.

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- Definition

Number of equity method investments

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No definition available.

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- Definition

Number of hotel rooms in a hotel

+ References

No definition available.

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- Definition

Number of stories in a high-rise building

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No definition available.

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- Definition

Supply Commitment, Remaining Maximum Amount Committed

+ References

No definition available.

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- Definition

For an unclassified balance sheet, this element represents costs incurred by the lessor that are (a) costs to originate a lease incurred in transactions with independent third parties that (i) result directly from and are essential to acquire that lease and (ii) would not have been incurred had that leasing transaction not occurred and (b) certain costs directly related to specified activities performed by the lessor for that lease. Those activities are: evaluating the prospective lessee's financial condition; evaluating and recording guarantees, collateral, and other security arrangements; negotiating lease terms; preparing and processing lease documents; and closing the transaction. This amount is before considering accumulated amortization representing the periodic charge to earnings to recognize the deferred costs over the term of the related lease.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 25

-Paragraph 17

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-SubTopic 20

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- Definition

The current amount of expenditures for a real estate project that has not yet been completed.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.10)

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

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- Definition

Net rentable area for properties owned.

+ References

No definition available.

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- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

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- Definition

The floor amount as of the balance sheet date that the entity must expend to satisfy the terms of disclosed arrangements (excluding long-term commitments) in which the entity must commit resources to supply goods or services to one or more customers.

+ 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 210.5-02.25)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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