Fair Value Measurements - Hierarchy (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Transfers between Levels 1 and 2
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0	$ 0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0	0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	0	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount	0	0
Transfers into and out of Level 3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0	0
Recurring
Financial Assets:
Total financial assets	109,724,000	141,637,000
Current Liabilities:
Warrants	2,644,000	3,792,000
Total financial liabilities	2,644,000	3,792,000
Recurring | Money market deposits
Financial Assets:
Total financial assets	3,477,000	10,777,000
Recurring | U.S. Treasury securities
Financial Assets:
Total financial assets	67,026,000	57,442,000
Recurring | U.S. agency securities
Financial Assets:
Total financial assets	27,072,000	56,114,000
Recurring | Municipal securities
Financial Assets:
Total financial assets	12,149,000	17,304,000
Recurring | Level 1
Financial Assets:
Total financial assets	70,503,000	68,219,000
Recurring | Level 1 | Money market deposits
Financial Assets:
Total financial assets	3,477,000	10,777,000
Recurring | Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	67,026,000	57,442,000
Recurring | Level 2
Financial Assets:
Total financial assets	39,221,000	73,418,000
Recurring | Level 2 | U.S. agency securities
Financial Assets:
Total financial assets	27,072,000	56,114,000
Recurring | Level 2 | Municipal securities
Financial Assets:
Total financial assets	12,149,000	17,304,000
Recurring | Level 3
Current Liabilities:
Warrants	2,644,000	3,792,000
Total financial liabilities	$ 2,644,000	$ 3,792,000