Financial Instruments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financial Instruments
Transfers between fair value measurement levels	$ 0		$ 0
Cash and cash equivalents:
Cash and cash equivalents	19,876	$ 16,369	$ 21,713
Short-term investments:
Short-term investments	23,363	0
Corporate debt securities
Short-term investments:
Short-term investments	$ 10,363
Government Securities and Obligations | Reverse Repurchase Agreement | Minimum
Financial Instruments
Percentage of investment amount to collateralized deposits value	102.00%
Recurring
Assets:
Total assets	$ 37,034	13,588
Recurring | Money market funds
Cash and cash equivalents:
Cash and cash equivalents	9,427	13,588
Recurring | Corporate debt securities
Cash and cash equivalents:
Cash and cash equivalents	1,244
Short-term investments:
Short-term investments	10,363
Recurring | U.S. reverse repurchase agreements
Cash and cash equivalents:
Cash and cash equivalents	3,000
Short-term investments:
Short-term investments	13,000
Recurring | Level 1
Assets:
Total assets	9,427	13,588
Recurring | Level 1 | Money market funds
Cash and cash equivalents:
Cash and cash equivalents	9,427	$ 13,588
Recurring | Level 2
Assets:
Total assets	27,607
Recurring | Level 2 | Corporate debt securities
Cash and cash equivalents:
Cash and cash equivalents	1,244
Short-term investments:
Short-term investments	10,363
Recurring | Level 2 | U.S. reverse repurchase agreements
Cash and cash equivalents:
Cash and cash equivalents	3,000
Short-term investments:
Short-term investments	$ 13,000