Fair Value of Financial Instruments - Financial Instruments Outstanding (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Estimated fair values of financial instruments
Cash and cash equivalents, cost	$ 3,868	$ 8,766	$ 5,640	$ 3,110
Cash and cash equivalents, fair value	$ 3,868	8,766
Short-term investments.
Estimated fair values of financial instruments
Total		1,002
Fair Value		1,002
Short-term investments. | U.S. government-sponsored enterprise bonds
Estimated fair values of financial instruments
Total		762
Fair Value		762
Short-term investments. | Corporate notes
Estimated fair values of financial instruments
Total		240
Fair Value		$ 240