Real Estate Investments - Summary of Properties Sold (Details) $ in Thousands			3 Months Ended		6 Months Ended
	Jun. 15, 2020 USD ($) a	Mar. 19, 2020 USD ($) ft²	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from the sale of properties					$ 21,250	$ 5,523
Gain (loss) on sale			$ 0	$ 0	$ 2,586	$ 0
Office properties | 52 East Swedesford Road
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable area (in square feet) | ft²		131,077
Proceeds from the sale of properties		$ 18,000
Gain (loss) on sale		$ 2,336
Office properties | Keith Valley
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from the sale of properties	$ 4,000
Gain (loss) on sale	$ 201
Land | Keith Valley
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area of land (in acres) | a	14.0