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Summary of Significant Accounting Policies - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Assets:    
U.S. treasury securities $ 19,900 $ 29,900
Level 1    
Assets:    
Money market funds (included in cash and cash equivalents) 21,957 32,936
Total assets 44,392 62,811
Liabilities:    
Common Stock warrants (included in accrued and other liabilities) 0 0
Level 3    
Assets:    
Money market funds (included in cash and cash equivalents) 0 0
Total assets 0 0
Liabilities:    
Common Stock warrants (included in accrued and other liabilities) 82 82
Cash and Cash Equivalents | Level 1    
Assets:    
U.S. treasury securities 2,495 0
Cash and Cash Equivalents | Level 3    
Assets:    
U.S. treasury securities 0 0
Short-term Investments | Level 1    
Assets:    
U.S. treasury securities 19,940 29,875
Short-term Investments | Level 3    
Assets:    
U.S. treasury securities $ 0 $ 0