Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Parenthetical) (Details)	Sep. 30, 2017 property
Segment Reporting Information [Line Items]
Number of Properties	97
Assets Held-for-sale [Member]
Segment Reporting Information [Line Items]
Number of Properties	6
Office Properties [Member]
Segment Reporting Information [Line Items]
Number of Properties	82
Office Properties [Member] | Pennsylvania Suburbs [Member] | Assets Held-for-sale [Member]
Segment Reporting Information [Line Items]
Number of Properties	6