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Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Segment Reporting Information [Line Items] |  |  |
Operating properties |  | $ 3,864,706 | $ 3,832,348
Construction-in-progress |  | 129,413 | 121,188
Land held for development | [1] | 99,436 | 98,242
Philadelphia CBD [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Operating properties |  | 1,653,373 | 1,643,296
Pennsylvania Suburbs [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Operating properties |  | 978,838 | 958,796
Metropolitan DC [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Operating properties |  | 979,041 | 978,257
Austin, Texas [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Operating properties |  | 163,788 | 163,653
Other [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Operating properties |  | $ 89,666 | $ 88,346
[1] | As of December 31, 2017, the Company categorized 13.1 acres of land held for development, located in the Other segment, as held for sale in accordance with applicable accounting standards for long lived assets.

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- Definition The current amount of expenditures for a real estate project that has not yet been completed. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.10) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Carrying amount of land available for development. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),1(d)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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