Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Fair value measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair value measurements
Marketable securities	$ 75,605	$ 98,625
Transfers between fair value measurement level 1 to level 2	0		$ 0
Transfers between fair value measurement level 2 to level 1	0		$ 0
Recurring basis
Fair value measurements
Cash and cash equivalents	32,354	26,591
Total assets	107,959	125,216
U.S. Treasuries \| Recurring basis
Fair value measurements
Marketable securities	59,225	62,640
Commercial paper \| Recurring basis
Fair value measurements
Marketable securities	9,989	24,931
Corporate bonds \| Recurring basis
Fair value measurements
Marketable securities	6,391	11,054
Quoted Prices in Active Markets (Level 1) \| Recurring basis
Fair value measurements
Cash and cash equivalents	32,354	26,591
Total assets	91,579	89,231
Quoted Prices in Active Markets (Level 1) \| U.S. Treasuries \| Recurring basis
Fair value measurements
Marketable securities	59,225	62,640
Significant Other Observable Inputs (Level 2) \| Recurring basis
Fair value measurements
Total assets	16,380	35,985
Significant Other Observable Inputs (Level 2) \| Commercial paper \| Recurring basis
Fair value measurements
Marketable securities	9,989	24,931
Significant Other Observable Inputs (Level 2) \| Corporate bonds \| Recurring basis
Fair value measurements
Marketable securities	$ 6,391	$ 11,054