Cash, cash equivalents, short-term investments and restricted cash - Short-term Investments and Restricted Cash and Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Cash and cash equivalents:
Cash	$ 61,396	$ 58,185
Cash equivalents	0	0
Cash and cash equivalents	61,396	58,185
Short-term investments	7,607	7,481
Restricted cash	10,827	11,053
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 72,223	$ 69,238	$ 57,294	$ 81,564