Real Estate Properties - Properties Held For Sale (Details) $ in Thousands	Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($)
Long Lived Assets Held-for-sale [Line Items]
Liabilities related to properties held for sale	$ 0	$ 1,840
Disposal Group, Held-for-sale, Not Discontinued Operations
Long Lived Assets Held-for-sale [Line Items]
Number of properties \| property	0
Rents receivable, allowance		5
Disposal Group, Held-for-sale, Not Discontinued Operations \| 1600 Market Street
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

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