Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Parenthetical) (Details)	9 Months Ended
	Sep. 30, 2018 a property	Dec. 31, 2017 a
Segment Reporting Information [Line Items]
Number of properties | property	93
3025 and 3001-3003 JFK Boulevard [Member]
Segment Reporting Information [Line Items]
Lease agreement term	99 years
Assets Held-for-sale [Member] | Land Held For Development [Member]
Segment Reporting Information [Line Items]
Acreage of land | a	37.9
Assets Held-for-sale [Member] | Pennsylvania Suburbs [Member] | Land Held For Development [Member]
Segment Reporting Information [Line Items]
Acreage of land | a	2.7
Assets Held-for-sale [Member] | Other [Member] | Land Held For Development [Member]
Segment Reporting Information [Line Items]
Acreage of land | a	35.2	13.1
Office Properties [Member]
Segment Reporting Information [Line Items]
Number of properties | property	73
Office Properties [Member] | Assets Held-for-sale [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Number of properties | property	8