Accounting for the Impairment or Disposal of Long-Lived Assets (Asset Impairments in Reportable Segments) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Asset impairment charges	$ 39,688	$ 10,632	$ 3,879
Impairment - real estate assets	32,431	3,946	3,879
Florida
Segment Reporting Information [Line Items]
Asset impairment charges	2,396	0	0
Houston
Segment Reporting Information [Line Items]
Asset impairment charges	0	10,632	0
Chicago/Minneapolis
Segment Reporting Information [Line Items]
Asset impairment charges	2,456	0	3,879
Southeastern PA
Segment Reporting Information [Line Items]
Asset impairment charges	1,579	0	0
Other
Segment Reporting Information [Line Items]
Asset impairment charges	7,257	0	0
Philadelphia
Segment Reporting Information [Line Items]
Asset impairment charges	26,000	0	$ 0
Discontinued operations
Segment Reporting Information [Line Items]
Asset impairment charges	7,300	6,700
Land and Land Improvements \| impairment - real estate assets
Segment Reporting Information [Line Items]
Asset impairment charges	6,400	$ 3,900
Land Held for Development \| Philadelphia
Segment Reporting Information [Line Items]
Impairment - real estate assets	$ 26,000

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2420-110228

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The charge against earnings in the period to reduce the carrying amount of real property to fair value.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109226691&loc=d3e2921-110230

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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