Summary of significant accounting policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Cash, cash equivalents and restricted cash
Cash and cash equivalents	$ 58,196	$ 59,634	$ 61,604	$ 26,591
Restricted cash included in other assets	321	371	371	371
Total cash, cash equivalents and restricted cash per statement of cash flows	$ 58,517	$ 60,005	$ 61,975	$ 26,962
Restricted cash, Balance Sheet Location	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent