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WARRANT DERIVATIVE LIABILITY (Tables)
 12 Months Ended
Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]  
Warrant Derivative Liability Rollforward
The Company did not have any investment securities classified as available-for-sale as of December 31, 2019. Investment securities classified as available-for-sale as of December 31, 2018 are presented in the below table:

(in thousands)
 
Amortized Cost
 
Gross Unrealized Gains
 
Gross Unrealized Losses
 
Aggregate Estimated Fair Value
December 31, 2018
 
 
 
 
 
 
 
 
U.S. government agency-backed securities
 
$
7,382

 
$
2

 
$
(1
)
 
$
7,383

Corporate debt securities
 
41,968

 
—

 
(47
)
 
41,921

Total
 
$
49,350

 
$
2

 
$
(48
)
 
$
49,304

The fair value of the warrants has been estimated with the following weighted-average assumptions:
 
 
December 31, 2019
 
September 30, 2019
 
August 21, 2019
Risk-free interest rate
 
1.83
%
 
1.62
%
 
1.54
%
Volatility
 
78.67
%
 
70.89
%
 
69.72
%
Expected life (years)
 
6.64

 
6.89

 
7.00

The following table reflects the fair value roll forward reconciliation of the warrant derivative liability for the year ended December 31, 2019:
(in thousands)
 
Warrant Derivative Liability
Fair value of Series 1 warrants at the date of issuance, August 21, 2019
 
$
32,992

Change in fair value
 
19,192

Fair value at December 31, 2019
 
$
52,184