| Summary of Significant Accounting Policies - Revenue from contract with customers (Details) - GlaxoSmithKline Intellectual Property Development Ltd - Collaboration and License Agreement £ in Millions | 1 Months Ended | 12 Months Ended | 16 Months Ended | 55 Months Ended | |||
|---|---|---|---|---|---|---|---|
| Sep. 30, 2018  USD ($) | Sep. 30, 2017  GBP (£) | Sep. 30, 2017  USD ($) | Jun. 30, 2014  USD ($) | Dec. 31, 2018  USD ($)  contract | Dec. 31, 2018  USD ($) | Dec. 31, 2018  USD ($) | |
| Collaboration and License Agreement | |||||||
| Number of contracts with customers | contract | 1 | ||||||
| Option Exercise Fee, received | £ 20 | $ 26,600,000 | |||||
| Upfront payment received | $ 42,123,000 | ||||||
| Option exercise fee received | $ 26,610,000 | ||||||
| Option exercise fee | $ 13,175,000 | $ 39,785,000 | |||||
| Amount of development milestone achieved | $ 66,404,000 | ||||||
| Estimate of variable consideration | $ 0 | ||||||
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- Definition Represents cash received upon achievement of development milestones. No definition available. 
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- Definition Represents the variable consideration entitled upon achievement of development milestones. No definition available. 
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- Definition The amount of first tranche of the option exercise fee receivable for exercising of agreement. No definition available. 
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- Definition Represents the total number of contracts with customers. No definition available. 
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- Definition Option exercise fee No definition available. 
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- Definition Option exercise fee received. No definition available. 
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- Definition Upfront payment received. No definition available. 
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