Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Parenthetical) (Details) - BDN - Al Venture [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2017
Segment Reporting Information [Line Items]
Equity method investment other-than-temporary impairment charge	$ 4.8	$ 4.8
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Equity method investment other-than-temporary impairment charge	$ 4.8	$ 4.8

This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 35
-Paragraph 32
-URI http://asc.fasb.org/extlink&oid=99375318&loc=d3e32787-111569

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration