Cash, cash equivalents, short-term investments and restricted cash - Maturities of marketable securities and additional information (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Less than 1 year	$ 7,481	$ 12,311
Due in 1 to 2 years	0	0
Due in 3 to 5 years	0	0
Total	$ 7,481	$ 12,311