| Quarterly Financial Data (Unaudited) | 
| (14) | Quarterly Financial Data (unaudited)   |    
|   |   | Three months ended |   |  
|   |   | December 31, 2019 |   |   | September 30, 2019 |   |   | June 30, 2019 |   |   | March 31, 2019 |   |  
| Revenue |   | $ | — |   |   | $ | — |   |   | $ | — |   |   | $ | 37 |   |  
| Total operating expenses |   |   | (23,178 | ) |   |   | (30,999 | ) |   |   | (27,378 | ) |   |   | (20,503 | ) |  
| Net loss and comprehensive loss |   |   | (23,641 | ) |   |   | (31,271 | ) |   |   | (27,638 | ) |   |   | (20,580 | ) |  
| Net loss attributable to ordinary shareholders |   |   | (23,641 | ) |   |   | (31,271 | ) |   |   | (27,638 | ) |   |   | (20,580 | ) |  
| Net loss per share attributable to ordinary shareholders – basic and diluted |   | $ | (1.59 | ) |   | $ | (2.15 | ) |   | $ | (1.93 | ) |   | $ | (1.44 | ) |  
| Weighted average ordinary shares outstanding – basic and diluted |   |   | 14,866,838 |   |   |   | 14,571,278 |   |   |   | 14,340,231 |   |   |   | 14,290,437 |   |    
|   |   | Three months ended |   |  
|   |   | December 31, 2018 |   |   | September 30, 2018 |   |   | June 30, 2018 |   |   | March 31, 2018 |   |  
| Revenue |   | $ | 239 |   |   | $ | 254 |   |   | $ | 185 |   |   | $ | 191 |   |  
| Total operating expenses |   |   | (24,183 | ) |   |   | (25,240 | ) |   |   | (15,611 | ) |   |   | (12,394 | ) |  
| Net loss and comprehensive loss |   |   | (24,258 | ) |   |   | (24,905 | ) |   |   | (15,747 | ) |   |   | (12,146 | ) |  
| Net loss attributable to ordinary shareholders |   |   | (24,258 | ) |   |   | (24,905 | ) |   |   | (15,747 | ) |   |   | (12,146 | ) |  
| Net loss per share attributable to ordinary shareholders – basic and diluted |   | $ | (1.72 | ) |   | $ | (1.77 | ) |   | $ | (2.22 | ) |   | $ | (61.36 | ) |  
| Weighted average ordinary shares outstanding – basic and diluted |   |   | 14,108,604 |   |   |   | 14,034,631 |   |   |   | 7,085,655 |   |   |   | 197,949 |   |    |