Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Parenthetical) (Details) - a	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
3025 and 3001-3003 JFK Boulevard [Member]
Segment Reporting Information [Line Items]
Lease agreement term	99 years
Pennsylvania Suburbs [Member] | Assets Held-for-sale [Member]
Segment Reporting Information [Line Items]
Acreage of land	2.7
Pennsylvania Suburbs [Member] | Assets Held-for-sale [Member] | Land Held For Development [Member]
Segment Reporting Information [Line Items]
Acreage of land	2.7
Other [Member] | Assets Held-for-sale [Member] | Land Held For Development [Member]
Segment Reporting Information [Line Items]
Acreage of land		13.1