Fair Value of Financial Instruments - Financial Instruments Outstanding (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Estimated fair values of financial instruments
Cash and cash equivalents, cost	$ 5,330	$ 8,766	$ 10,503	$ 5,640
Cash and cash equivalents, fair value	5,330	8,766
Short-term investments.
Estimated fair values of financial instruments
Total	240	1,002
Fair Value	240	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	240
Fair Value	$ 240	$ 240