Summary of significant accounting policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Cash, cash equivalents and restricted cash
Cash and cash equivalents	$ 60,450	$ 62,351	$ 137,279	$ 59,634
Restricted cash included in other assets, noncurrent	321	321	371	371
Total cash, cash equivalents and restricted cash per statement of cash flows	$ 60,771	$ 62,672	$ 137,650	$ 60,005
Restricted cash, Balance Sheet Location	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent