Fair Value Measurements - Hierarchy (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
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	135,303,000	$ 141,637,000
Current Liabilities:
Warrants	4,189,000	3,792,000
Total financial liabilities	4,189,000	3,792,000
Recurring | Money market deposits
Financial Assets:
Total financial assets	286,000	10,777,000
Recurring | U.S. Treasury securities
Financial Assets:
Total financial assets	84,279,000	57,442,000
Recurring | U.S. agency securities
Financial Assets:
Total financial assets	38,970,000	56,114,000
Recurring | Municipal securities
Financial Assets:
Total financial assets	11,768,000	17,304,000
Recurring | Level 1
Financial Assets:
Total financial assets	84,565,000	68,219,000
Recurring | Level 1 | Money market deposits
Financial Assets:
Total financial assets	286,000	10,777,000
Recurring | Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	84,279,000	57,442,000
Recurring | Level 2
Financial Assets:
Total financial assets	50,738,000	73,418,000
Recurring | Level 2 | U.S. agency securities
Financial Assets:
Total financial assets	38,970,000	56,114,000
Recurring | Level 2 | Municipal securities
Financial Assets:
Total financial assets	11,768,000	17,304,000
Recurring | Level 3
Current Liabilities:
Warrants	4,189,000	3,792,000
Total financial liabilities	$ 4,189,000	$ 3,792,000