Fair Value Measurements - Fair Value Hierarchy (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets
Total assets	$ 89,264	$ 86,719
Liabilities
Stock warrant liability		105
Total liabilities		105
Fair Value, Inputs, Level 1
Assets
Total assets	149	13,922
Fair Value, Inputs, Level 2
Assets
Total assets	89,115	72,797
Fair Value, Inputs, Level 3
Liabilities
Stock warrant liability		105
Total liabilities		105
Money market funds
Assets
Total assets	149	13,922
Money market funds | Fair Value, Inputs, Level 1
Assets
Total assets	149	13,922
U.S.government bonds
Assets
Total assets	1,498
U.S.government bonds | Fair Value, Inputs, Level 2
Assets
Total assets	1,498
U.S Government agency bonds
Assets
Total assets	7,273	5,523
U.S Government agency bonds | Fair Value, Inputs, Level 2
Assets
Total assets	7,273	5,523
Bank certificates of deposit
Assets
Total assets	12,255
Bank certificates of deposit | Fair Value, Inputs, Level 2
Assets
Total assets	12,255
Commercial paper.
Assets
Total assets	15,094	15,340
Commercial paper. | Fair Value, Inputs, Level 2
Assets
Total assets	15,094	15,340
Corporate notes
Assets
Total assets	46,341	45,159
Corporate notes | Fair Value, Inputs, Level 2
Assets
Total assets	46,341	45,159
Asset-backed securities
Assets
Total assets	6,654	6,775
Asset-backed securities | Fair Value, Inputs, Level 2
Assets
Total assets	$ 6,654	$ 6,775