Notes Payable - Contractual Obligations for Principal Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
2018	$ 52,932
2019	23,934
2020	13,384
2021	0
2022	0
Thereafter	0
Total contractual obligations for principal payments	90,250
Unamortized premium	71
Total debt	90,321	$ 143,119	$ 143,119
Deferred financing fees, net	(400)	(787)	(787)
Notes payable, net of deferred financing fees	$ 86,819	$ 89,921	$ 142,332