Financial Instruments not Reported at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Notes payable, carrying amount	$ 90,321	$ 143,119	$ 143,119
Less: unamortized debt issuance costs, carrying amounts	(400)	(787)	(787)
Notes payable, net, carrying amount	$ 86,819	89,921	142,332
Notes payable, fair value		$ 91,449	$ 146,790