Fair Value Measurements - Outstanding Debt Carrying Amount and Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest-bearing obligations	$ 14,322	$ 43,167
Carrying Amount [Member] | Hercules Term Loan [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest-bearing obligations		16,850
Carrying Amount [Member] | Novartis Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest-bearing obligations	14,322	14,086
Carrying Amount [Member] | Servier Loan [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest-bearing obligations		12,231
Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest-bearing obligations	14,018	42,531
Estimated Fair Value [Member] | Hercules Term Loan [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest-bearing obligations		16,453
Estimated Fair Value [Member] | Novartis Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest-bearing obligations	$ 14,018	13,836
Estimated Fair Value [Member] | Servier Loan [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest-bearing obligations		$ 12,242