Cash, cash equivalents, restricted cash, and available for sale marketable securities - Restricted cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash, cash equivalents, restricted cash, and available for sale marketable securities
Cash and cash equivalents	$ 140,371	$ 31,530	$ 43,604
Restricted cash included in deposits and other noncurrent assets	735		735
Total cash, cash equivalents, and restricted cash	$ 141,106	$ 32,265	$ 44,339	$ 37,376