4. Investments

Our portfolio of fixed income securities consists of commercial paper and corporate debt securities. All our investments in fixed income securities are classified as held-to-maturity. These investments are carried at amortized cost.

We had no investments in fixed income securities as of December 31, 2019.

Our investments in fixed income securities as of December 31, 2018 were as follows:



	As of December 31, 2018
	Amortized						Fair
(in thousands)	cost		Gains		Losses		Value
Corporate debt securities	$	6,556	$	—	$	(1)	$	6,555
Commercial paper		53,600		1		(3)		53,598
Total	$	60,156	$	1	$	(4)	$	60,153

The following table summarizes the balance sheet classification of our investments as of December 31, 2018:


	As of December 31,
(in thousands)	2018
Cash equivalents	$	59,156
Short-term investments		1,000
Total investments	$	60,156

The gross amortized cost and estimated fair value of our held-to-maturity investments at December 31, 2018 by contractual maturity are shown below. Actual maturities may differ from contractual maturities because the issuers of the securities may have the right to prepay obligations without prepayment penalties.


	As of December 31,
	2018
	Gross
	Amortized		Fair
(in thousands)	Cost		Value
Due in one year or less	$	60,156	$	60,153
Due after one year through five years		—		—
Total	$	60,156	$	60,153

We monitor our investment portfolio for impairment on a periodic basis. In order to determine whether a decline in fair value is other-than-temporary, we evaluate, among other factors: the duration and extent to which the fair value has been less than the carrying value; our financial condition and business outlook, including key operational and cash flow metrics, current market conditions and future trends in our industry; our relative competitive position within the industry; and our intent and ability to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value. A decline in the fair value of the security below amortized cost that is deemed other-than-temporary is charged to earnings, resulting in the establishment of a new cost basis for the affected securities. In 2019, we had an insignificant amount of unrealized gains or losses, and we did not recognize any other-than-temporary impairments.