Fair Value Measurements - Hierarchy (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
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	141,637,000	207,552,000
Current Liabilities:
Warrants	3,792,000	3,649,000
Total financial liabilities	3,792,000	3,649,000
Recurring | Money market deposits
Financial Assets:
Total financial assets	10,777,000	5,966,000
Recurring | U.S. Treasury securities
Financial Assets:
Total financial assets	57,442,000	103,813,000
Recurring | U.S. agency securities
Financial Assets:
Total financial assets	56,114,000	78,853,000
Recurring | Municipal securities
Financial Assets:
Total financial assets	17,304,000	18,920,000
Recurring | Level 1
Financial Assets:
Total financial assets	68,219,000	109,779,000
Recurring | Level 1 | Money market deposits
Financial Assets:
Total financial assets	10,777,000	5,966,000
Recurring | Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	57,442,000	103,813,000
Recurring | Level 2
Financial Assets:
Total financial assets	73,418,000	97,773,000
Recurring | Level 2 | U.S. agency securities
Financial Assets:
Total financial assets	56,114,000	78,853,000
Recurring | Level 2 | Municipal securities
Financial Assets:
Total financial assets	17,304,000	18,920,000
Recurring | Level 3
Current Liabilities:
Warrants	3,792,000	3,649,000
Total financial liabilities	$ 3,792,000	$ 3,649,000