Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Parenthetical) (Details) - a	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Assets Held-for-sale [Member] | Land Held For Development [Member]
Segment Reporting Information [Line Items]
Acreage of Land		37.9
3001-3003 and 3025 JFK Boulevard [Member]
Segment Reporting Information [Line Items]
Lease agreement term	99 years	99 years
Other [Member] | Assets Held-for-sale [Member]
Segment Reporting Information [Line Items]
Acreage of Land	35.2
Other [Member] | Assets Held-for-sale [Member] | Land Held For Development [Member]
Segment Reporting Information [Line Items]
Acreage of Land	35.2	35.2
Pennsylvania Suburbs [Member] | Assets Held-for-sale [Member] | Land Held For Development [Member]
Segment Reporting Information [Line Items]
Acreage of Land		2.7