Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash and cash equivalents:
Cash and cash equivalents	$ 15,294	$ 16,369
Short-term investments:
Short-term investments	22,276	0
Money market funds
Cash and cash equivalents:
Cash and cash equivalents	5,956	13,588
Corporate debt securities
Cash and cash equivalents:
Cash and cash equivalents	1,948
Short-term investments:
Short-term investments	7,276
Reverse repurchase agreements
Cash and cash equivalents:
Cash and cash equivalents	3,000
Short-term investments:
Short-term investments	15,000
Recurring
Assets:
Total assets	33,180	13,588
Recurring | Money market funds
Cash and cash equivalents:
Cash and cash equivalents	5,956	13,588
Recurring | Corporate debt securities
Cash and cash equivalents:
Cash and cash equivalents	1,948
Short-term investments:
Short-term investments	7,276
Recurring | Reverse repurchase agreements
Cash and cash equivalents:
Cash and cash equivalents	3,000
Short-term investments:
Short-term investments	15,000
Recurring | Level 1
Assets:
Total assets	5,956	13,588
Recurring | Level 1 | Money market funds
Cash and cash equivalents:
Cash and cash equivalents	5,956	$ 13,588
Recurring | Level 2
Assets:
Total assets	27,224
Recurring | Level 2 | Corporate debt securities
Cash and cash equivalents:
Cash and cash equivalents	1,948
Short-term investments:
Short-term investments	7,276
Recurring | Level 2 | Reverse repurchase agreements
Cash and cash equivalents:
Cash and cash equivalents	3,000
Short-term investments:
Short-term investments	$ 15,000