Cash, cash equivalents, short-term investments and restricted cash - Maturities of marketable securities and additional information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) item	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) item
Cash, cash equivalents, short-term investments, and restricted cash
Less than 1 year	$ 26,115		$ 36,054
Due in 1 to 2 years	3,741		6,468
Due in 3 to 5 years			302
Total	29,856		$ 42,824
Impairment losses on its marketable securities	$ 0	$ 0
Investments in marketable securities in unrealized loss position in excess of 12 months | item	0		0