Fair Value Measurements - Hierarchy (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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
Current Liabilities:
Warrants		2,644,000
Total financial liabilities		2,644,000
Recurring | Available-for-sale securities
Financial Assets:
Total financial assets	134,000,000	109,724,000
Recurring | Money market deposits
Financial Assets:
Total financial assets	26,539,000	3,477,000
Recurring | U.S. Treasury securities
Financial Assets:
Total financial assets	75,685,000	67,026,000
Recurring | U.S. agency securities
Financial Assets:
Total financial assets	12,891,000	27,072,000
Recurring | Municipal securities
Financial Assets:
Total financial assets	18,885,000	12,149,000
Recurring | Level 1 | Available-for-sale securities
Financial Assets:
Total financial assets	102,224,000	70,503,000
Recurring | Level 1 | Money market deposits
Financial Assets:
Total financial assets	26,539,000	3,477,000
Recurring | Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	75,685,000	67,026,000
Recurring | Level 2 | Available-for-sale securities
Financial Assets:
Total financial assets	31,776,000	39,221,000
Recurring | Level 2 | U.S. agency securities
Financial Assets:
Total financial assets	12,891,000	27,072,000
Recurring | Level 2 | Municipal securities
Financial Assets:
Total financial assets	$ 18,885,000	12,149,000
Recurring | Level 3
Current Liabilities:
Warrants		2,644,000
Total financial liabilities		$ 2,644,000