Real Estate Investments (Acquisitions and Dispositions) (Details) (USD $) In Millions, unless otherwise specified	Jan. 06, 2012 Plymouth Meeting, Pennsylvania [Member] sqft	Jun. 22, 2012 Pacific Ridge Corporate Center [Member] sqft	Mar. 22, 2012 South Lake at Dulles Corner [Member] sqft	Jan. 17, 2012 304 Harper Drive [Member] sqft
Significant Acquisitions and Disposals [Line Items]
Net rentable space of property acquired	154,392
Acquired real estate, cost of property	$ 9.1
Acquisition related costs	0.1
Net rentable square feet of properties sold		121,381	268,240	32,978
Sales price of properties sold		$ 29.0	$ 91.1	$ 3.0
Lease percentage of sold property		83.70%	100.00%	90.10%

Amount of direct costs of the business combination including legal, accounting, and other costs incurred to consummate the business acquisition.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 51

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 24

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_BusinessAcquisitionCostOfAcquiredEntityTransactionCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant