Short-Term Investments (Details)	3 Months Ended
	Mar. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Short-term investments
Short-term Investments. | $	$ 3,338,000	$ 3,246,000
Certificates of Deposit
Short-term investments
Number of fully insured certificates of deposit | item	14
Certificates of Deposit | Minimum
Short-term investments
Maturity period	9 months
Interest rate (as a percent)	0.60%
Certificates of Deposit | Maximum
Short-term investments
Maturity period	12 months
Interest rate (as a percent)	0.95%