Cash, cash equivalents, short-term investments and restricted cash - Short term investments and restricted cash and investments (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Cash and cash equivalents:
Cash	$ 53,851	$ 58,691
Cash equivalents	19,810	23,809
Cash and cash equivalents	73,661	82,500	$ 82,476	$ 76,088
Short-term investments	2,055	19,015
Restricted cash and investments:
Restricted cash	3,290	4,085
Optoelectronics Components Co. Ltd.
Restricted cash and investments:
Restricted cash	$ 2,000	$ 2,000