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Short-term Investments
 12 Months Ended
Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]  
Short-term Investments Short-term Investments
The Company's investments are classified as available-for-sale. The following table summarizes the amortized cost, gross unrealized gains and losses, and fair value of available-for-sale investments as of December 31, 2019:
 
Amortized
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Aggregate
Fair Value
United States government debt securities
$
8,989

 
$
7

 
$
—

 
$
8,996

Certificates of deposit
264

 
—

 
—

 
264

Total
$
9,253

 
$
7

 
$
—

 
$
9,260


The following table summarizes the amortized cost, gross unrealized gains and losses, and fair value of available-for-sale investments as of December 31, 2018:
 
Amortized
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Aggregate
Fair Value
United States government debt securities
$
119,401

 
$
—

 
$
(61
)
 
$
119,340

Certificates of deposit
274

 
—

 
—

 
274

Total
$
119,675

 
$
—

 
$
(61
)
 
$
119,614


See Notes 2 and 8 for further discussion on the Company's method for determining the fair value of its assets.
As of December 31, 2019, all of the available-for-sale investments were due within one year based on their contractual maturities.
Changes in market interest rates and bond yields cause certain investments to fall below their cost basis, resulting in unrealized losses on investments. The unrealized losses of the Company's investments as of December 31, 2018 were primarily a result of unfavorable changes in interest rates subsequent to the initial purchase of these investments, and there were none as of December 31, 2019.
As of December 31, 2018, the Company did not consider any of its debt security investments to be other-than-temporarily impaired. When evaluating its debt security investments for other-than-temporary impairment, the Company reviews factors such as the length of time and extent to which fair value has been below its cost basis, the financial condition of the issuer, the Company's ability and intent to hold the security and whether it is more likely than not that it will be required to sell the investment before recovery of its cost basis.