Cash, cash equivalents, short-term investments and restricted cash - Maturities of marketable securities and additional information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Cash, cash equivalents, short-term investments, and restricted cash
Less than 1 year	$ 21,865		$ 21,865		$ 36,054
Due in 1 to 2 years					6,468
Due in 3 to 5 years					302
Total	21,865		21,865		$ 42,824
Impairment losses on its marketable securities	$ 0	$ 0	$ 0	$ 0
Investments in marketable securities in unrealized loss position in excess of 12 months | item	0		0