Significant Accounting Policies - Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Cash and cash equivalents	$ 19,959	$ 21,942	$ 26,989
Restricted cash	2,957	3,251	2,824
Cash, cash equivalents, and restricted cash	$ 22,916	$ 25,193	$ 29,813	$ 30,107