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Condensed consolidated financial statement details - Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Marketable securities	$ 24,951	$ 44,886
Cash	76,600	154,800
Cash and cash equivalents	114,843	202,504	$ 149,755	$ 192,114
Fair Value, Measurements, Recurring [Member]
Cash and Cash Equivalents [Line Items]
Marketable securities	$ 24,951	$ 44,886