Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash and cash equivalents:
Cash and cash equivalents	$ 23,596	$ 62,780	$ 14,668	$ 30,474
Available-for-Sale Securities:
Available-for-sale securities	14,931	0
Corporate debt securities
Available-for-Sale Securities:
Available-for-sale securities	13,930
U.S. government-sponsored securities
Available-for-Sale Securities:
Available-for-sale securities	1,001
Recurring
Assets:
Total assets	37,354	62,004
Recurring | Money market funds
Cash and cash equivalents:
Cash and cash equivalents	22,423	62,004
Recurring | Corporate debt securities
Available-for-Sale Securities:
Available-for-sale securities	13,930
Recurring | U.S. government-sponsored securities
Available-for-Sale Securities:
Available-for-sale securities	1,001
Recurring | Level 1
Assets:
Total assets	22,423	62,004
Recurring | Level 1 | Money market funds
Cash and cash equivalents:
Cash and cash equivalents	22,423	$ 62,004
Recurring | Level 2
Assets:
Total assets	14,931
Recurring | Level 2 | Corporate debt securities
Available-for-Sale Securities:
Available-for-sale securities	13,930
Recurring | Level 2 | U.S. government-sponsored securities
Available-for-Sale Securities:
Available-for-sale securities	$ 1,001