Fair Value Measurements - Hierarchy (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Transfers between Levels 1 and 2
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount	0
Recurring
Financial Assets:
Total financial assets	148,771,000	$ 109,724,000
Current Liabilities:
Warrants		2,644,000
Total financial liabilities		2,644,000
Recurring | Money market deposits
Financial Assets:
Total financial assets	17,880,000	3,477,000
Recurring | U.S. Treasury securities
Financial Assets:
Total financial assets	100,468,000	67,026,000
Recurring | U.S. agency securities
Financial Assets:
Total financial assets	18,849,000	27,072,000
Recurring | Municipal securities
Financial Assets:
Total financial assets	11,574,000	12,149,000
Recurring | Level 1
Financial Assets:
Total financial assets	118,348,000	70,503,000
Recurring | Level 1 | Money market deposits
Financial Assets:
Total financial assets	17,880,000	3,477,000
Recurring | Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	100,468,000	67,026,000
Recurring | Level 2
Financial Assets:
Total financial assets	30,423,000	39,221,000
Recurring | Level 2 | U.S. agency securities
Financial Assets:
Total financial assets	18,849,000	27,072,000
Recurring | Level 2 | Municipal securities
Financial Assets:
Total financial assets	$ 11,574,000	12,149,000
Recurring | Level 3
Current Liabilities:
Warrants		2,644,000
Total financial liabilities		$ 2,644,000