Fair Value Measurements - Fair Value Hierarchy (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets
Total assets	$ 89,348	$ 86,719
Liabilities
Stock warrant liability		105
Total liabilities		105
Fair Value, Inputs, Level 1
Assets
Total assets	80	13,922
Fair Value, Inputs, Level 2
Assets
Total assets	89,268	72,797
Fair Value, Inputs, Level 3
Liabilities
Stock warrant liability		105
Total liabilities		105
Money market funds
Assets
Total assets	80	13,922
Money market funds | Fair Value, Inputs, Level 1
Assets
Total assets	80	13,922
U.S. Government agency bonds
Assets
Total assets	9,515	5,523
U.S. Government agency bonds | Fair Value, Inputs, Level 2
Assets
Total assets	9,515	5,523
Bank certificates of deposit
Assets
Total assets	11,113
Bank certificates of deposit | Fair Value, Inputs, Level 2
Assets
Total assets	11,113
Commercial paper.
Assets
Total assets	17,010	15,340
Commercial paper. | Fair Value, Inputs, Level 2
Assets
Total assets	17,010	15,340
Corporate notes
Assets
Total assets	45,124	45,159
Corporate notes | Fair Value, Inputs, Level 2
Assets
Total assets	45,124	45,159
Asset-backed securities
Assets
Total assets	6,506	6,775
Asset-backed securities | Fair Value, Inputs, Level 2
Assets
Total assets	$ 6,506	$ 6,775