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Fair Value Measurements - Summary of Estimated Fair Value of Free Shares Valuation Assumptions (Detail)
Jun. 30, 2019
$ / shares
€ / shares
Dec. 31, 2018
$ / shares
€ / shares
Stock Price    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Free shares measurement input | $ / shares 9.65 11.48
Exchange Rate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Free shares measurement input 0.89 0.87
Stock Price Volatility Estimate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Free shares measurement input 77.6 79.9
EUR / USD Exchange Rate Volatility Estimate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Free shares measurement input 7.0 7.7
TxCell S.A. | Stock Price    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Free shares measurement input | € / shares 2.24 2.58
TxCell S.A. | Stock Price Correlation    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Free shares measurement input 72.2 0
TxCell S.A. | Stock Price Volatility Estimate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Free shares measurement input 76.2 8.6