Commitments and Contingencies- Ground Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leased Assets [Line Items]
Rent expense ground lease	$ 160	$ 154	$ 18
Kerrow Subsidiary
Operating Leased Assets [Line Items]
Rent expense ground lease	$ 448	$ 466	$ 441