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Fair Value of Financial Assets and Liabilities - (Details) - USD ($)
$ / shares in Units, $ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Marketable securities:    
Shares that warrant can purchase 7,614  
Warrant purchase price $ 3.94  
Recurring    
Assets:    
Money market funds, included in cash and cash equivalents $ 71,581 $ 114,593
Marketable securities:    
U.S. Treasury obligations 71,333 60,576
Total assets 142,914 175,169
Recurring | Level 1    
Assets:    
Money market funds, included in cash and cash equivalents 71,581 114,593
Marketable securities:    
U.S. Treasury obligations 71,333 60,576
Total assets $ 142,914 $ 175,169