Real Estate Investments - Summary of Properties Classified as Held for Sale Included in Continuing Operations (Parenthetical) (Details)	Sep. 30, 2017 ft² property
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties	97
Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties	82
Held for Sale Properties Included in Continuing Operations [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties	6
Held for Sale Properties Included in Continuing Operations [Member] | Pennsylvania Suburbs [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties	6
Held for Sale Properties Included in Continuing Operations [Member] | Pennsylvania Suburbs [Member] | 630 Allendale Road - Office [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²	150,000
Held for Sale Properties Included in Continuing Operations [Member] | Pennsylvania Suburbs [Member] | Newtown Square - Office [Member] | Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Rentable Square Feet | ft²	252,802
Number of Properties	5