Fair Value of Financial Instruments - Summary (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Short-term investments:
Corporate commercial paper, bonds and notes	$ 6,652	$ 634
Corporate commercial paper, bonds and notes
Short-term investments:
Corporate commercial paper, bonds and notes	6,652	634
Fair Value, Measurements, Recurring
Short-term investments:
Total assets at fair value	26,647	21,148
Fair Value, Measurements, Recurring | Corporate commercial paper, bonds and notes
Short-term investments:
Corporate commercial paper, bonds and notes	6,652	634
Fair Value, Measurements, Recurring | Money market funds
Cash equivalents:
Cash equivalents	19,605	18,180
Fair Value, Measurements, Recurring | Corporate Commercial Paper, Stock, Bonds And Notes | Corporate commercial paper, bonds and notes
Cash equivalents:
Cash equivalents	300	2,199
Fair Value, Measurements, Recurring | Municipal bonds
Cash equivalents:
Cash equivalents	90	135
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Short-term investments:
Total assets at fair value	19,605	18,180
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Corporate commercial paper, bonds and notes
Short-term investments:
Corporate commercial paper, bonds and notes	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Money market funds
Cash equivalents:
Cash equivalents	19,605	18,180
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Corporate Commercial Paper, Stock, Bonds And Notes | Corporate commercial paper, bonds and notes
Cash equivalents:
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Municipal bonds
Cash equivalents:
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Other Observable Inputs (Level 2)
Short-term investments:
Total assets at fair value	7,042	2,968
Fair Value, Measurements, Recurring | Other Observable Inputs (Level 2) | Corporate commercial paper, bonds and notes
Short-term investments:
Corporate commercial paper, bonds and notes	6,652	634
Fair Value, Measurements, Recurring | Other Observable Inputs (Level 2) | Money market funds
Cash equivalents:
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Other Observable Inputs (Level 2) | Corporate Commercial Paper, Stock, Bonds And Notes | Corporate commercial paper, bonds and notes
Cash equivalents:
Cash equivalents	300	2,199
Fair Value, Measurements, Recurring | Other Observable Inputs (Level 2) | Municipal bonds
Cash equivalents:
Cash equivalents	90	135
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3)
Short-term investments:
Total assets at fair value	0	0
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) | Corporate commercial paper, bonds and notes
Short-term investments:
Corporate commercial paper, bonds and notes	0	0
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) | Money market funds
Cash equivalents:
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) | Corporate Commercial Paper, Stock, Bonds And Notes | Corporate commercial paper, bonds and notes
Cash equivalents:
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) | Municipal bonds
Cash equivalents:
Cash equivalents	$ 0	$ 0