Real Estate Properties - Properties Held For Sale (Details) $ in Thousands	1 Months Ended
	Feb. 28, 2018 USD ($) ft² building property	Jan. 31, 2013 building	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Long Lived Assets Held-for-sale [Line Items]
Number of Buildings | building		18
Liabilities related to properties held for sale			$ 1,840	$ 0
Disposal Group, Held-for-sale, Not Discontinued Operations
Long Lived Assets Held-for-sale [Line Items]
Real estate properties			76,066
Rents receivable, net of allowance for doubtful accounts of $5			13,270
Other assets, net			8,352
Assets held for sale			97,688
Accounts payable and accrued expenses			1,021
Rent collected in advance			408
Security deposits			411
Liabilities related to properties held for sale			1,840
Rents receivable, allowance			$ 5
Subsequent event | 1600 Market Street | Disposal Group, Held-for-sale, Not Discontinued Operations
Long Lived Assets Held-for-sale [Line Items]
Number of Properties | property	1
Number of Buildings | building	1
Square Footage (in sqft) | ft²	825,968
Gross Sales Price	$ 160,000