Acquisitions (Details)	Dec. 12, 2018 USD ($) ft²	Oct. 25, 2018 USD ($) ft²	Feb. 23, 2018 USD ($) ft²	Dec. 31, 2018 USD ($) ft² building
Business Acquisition [Line Items]
Number of properties acquired | building				3
Rentable square feet | ft²				16,200,000
501 West Church Street
Business Acquisition [Line Items]
Ownership Percentage Acquired			100.00%
Rentable square feet | ft²			182,461
Percentage Leased as of Acquisition (Unaudited)			100.00%
Net Contractual Purchase Price (in millions) | $			$ 28,000,000.0
9320 Excelsior Boulevard
Business Acquisition [Line Items]
Ownership Percentage Acquired		100.00%
Rentable square feet | ft²		267,724
Percentage Leased as of Acquisition (Unaudited)		100.00%
Net Contractual Purchase Price (in millions) | $		$ 48,700,000
25 Burlington Mall Road
Business Acquisition [Line Items]
Ownership Percentage Acquired	100.00%
Rentable square feet | ft²	287,776
Percentage Leased as of Acquisition (Unaudited)	89.00%
Net Contractual Purchase Price (in millions) | $	$ 74,000,000.0
Unsecured Debt | Line of Credit | Unsecured 500 Million 2018 Line Of Credit
Business Acquisition [Line Items]
Amount of line of credit | $				$ 500,000,000