Short-Term Investments (Details)	6 Months Ended
	Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Short-term investments
Short-term Investments. | $	$ 2,920,000	$ 2,895,000
Certificates of Deposit
Short-term investments
Number of fully insured certificates of deposit | item	12
Certificates of Deposit | Minimum
Short-term investments
Maturity period	5 months
Interest rate (as a percent)	1.55%
Certificates of Deposit | Maximum
Short-term investments
Maturity period	6 months
Interest rate (as a percent)	2.00%