Segment Information (Narrative) (Details) - Operating Segments	Sep. 30, 2017
Other \| Fashion Centre Mall / Washington Tower
Segment Information
Equity method ownership percentage	7.50%
Other \| Rosslyn Plaza \| Minimum
Segment Information
Equity method ownership percentage	43.70%
Other \| Rosslyn Plaza \| Maximum
Segment Information
Equity method ownership percentage	50.40%
New York \| 85 Tenth Avenue
Segment Information
Equity method ownership percentage	49.90%

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant