Fair Value Measurements - Hierarchy (Details) - Recurring - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Financial Assets:
Total financial assets	$ 207,485	$ 207,552
Current Liabilities:
Warrants	3,955	3,649
Total financial liabilities	3,955	3,649
Money market deposits
Financial Assets:
Total financial assets	8,838	5,966
U.S. Treasury securities
Financial Assets:
Total financial assets	102,820	103,813
U.S. agency securities
Financial Assets:
Total financial assets	77,903	78,853
Municipal securities
Financial Assets:
Total financial assets	17,924	18,920
Level 1
Financial Assets:
Total financial assets	111,658	109,779
Level 1 | Money market deposits
Financial Assets:
Total financial assets	8,838	5,966
Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	102,820	103,813
Level 2
Financial Assets:
Total financial assets	95,827	97,773
Level 2 | U.S. agency securities
Financial Assets:
Total financial assets	77,903	78,853
Level 2 | Municipal securities
Financial Assets:
Total financial assets	17,924	18,920
Level 3
Current Liabilities:
Warrants	3,955	3,649
Total financial liabilities	$ 3,955	$ 3,649