Summary of Significant Accounting Policies - Cash and cash equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Cash and cash equivalents	$ 6,215	$ 5,409
Restricted cash	0	49,098
Total cash, cash equivalents and restricted cash	$ 6,215	$ 54,507	$ 4,102	$ 3,519