Cash and Cash Equivalents and Restricted Cash - Cash and Cash Equivalents by Geographic Region (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 59,433	$ 62,438
United States
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	6,465	10,489
People's Republic of China (PRC)
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	34,710	23,283
Asia (excluding the PRC)
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	1,348	1,405
Europe
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	8,139	18,071
South America
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 8,771	$ 9,190