Organization and Basis of Presentation - Ground Leases (Details)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) properties	Jun. 30, 2019 USD ($) properties	Dec. 31, 2018 USD ($)
Lessee, Lease, Description [Line Items]
Number of properties subject to ground lease (in properties) | properties	5	5
Weighted average remaining lease term	47 years	47 years
Term of contract	11 years	11 years
Useful life increase interval		5 years
Weighted average discount rate	4.60%	4.60%
Operating Lease, Expense	$ 482,000	$ 950,000
Leasing expense			$ 1,500,000
Cash portion of lease expense	$ 453,000	$ 886,000
Minimum
Lessee, Lease, Description [Line Items]
Renewal term	25 years	25 years
Weighted average discount rate	4.10%	4.10%
Maximum
Lessee, Lease, Description [Line Items]
Term of contract	110 years	110 years
Renewal term	48 years	48 years
Weighted average discount rate	5.00%	5.00%
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Leasing expense	$ 300,000	$ 600,000