Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Additional Information) (Details) - a	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Assets Held-for-sale | Land Held For Development
Segment Reporting Information [Line Items]
Area of land (in acres)		37.9
3001-3003 and 3025 JFK Boulevard
Segment Reporting Information [Line Items]
Lease agreement term	99 years	99 years
Other | Assets Held-for-sale
Segment Reporting Information [Line Items]
Area of land (in acres)	35.2
Other | Assets Held-for-sale | Land Held For Development
Segment Reporting Information [Line Items]
Area of land (in acres)	35.2	35.2
Pennsylvania Suburbs | Assets Held-for-sale | Land Held For Development
Segment Reporting Information [Line Items]
Area of land (in acres)		2.7