Fair Value Measurements - Outstanding Long Term Debt Carrying Amount and Estimated Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 22,583	$ 22,479
Carrying Amount [Member] | Novartis Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	15,193	15,193
Carrying Amount [Member] | SVB Loan [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	7,390	7,286
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	22,518	22,106
Fair Value [Member] | Novartis Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	15,130	14,825
Fair Value [Member] | SVB Loan [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 7,388	$ 7,281