Cash Equivalents — Cash equivalents consist of highly liquid investments with original or remaining maturities of 90 days or less at the time of purchase that are readily convertible into cash.

Available-for-Sale Investment — The Thirona convertible notes are classified as available-for-sale securities and are included in prepaid expenses and other current assets in the consolidated balance sheets. Available-for-sale investments are subsequently measured at fair value with realized gains and losses reported in other income (expense) in the consolidated statements of operations. Unrealized holding gains and losses are excluded from earnings and reported in other comprehensive income (loss) until realized. The Company determines the fair value of its available-for-sale investments using level 3 inputs and evaluates the fair value of its investment in Thirona by applying a Monte Carlo simulation model. For each of the years ended December 31, 2023 and 2022, the Company recognized $0.5 million of interest income on investment. The Company's investment in Thirona is comprised of two notes with aggregate face value of $8.0 million and stated interest rate of 6%. As of December 31, 2023 and 2022, the fair value of the Company's investment in Thirona was $6.9 million and $7.1 million, respectively. In addition, the Company determined that there were related credit losses on the investment of $0.2 million and $0.9 million during the years ended December 31, 2023 and 2022, respectively, which were recognized in the consolidated statements of operations. No unrealized holding gain or loss was recognized in accumulated other comprehensive income (loss) during the years ended December 31, 2023, 2022,- and 2021.

Held-to-Maturity Investments — Investments consist of highly liquid investments that are intended to facilitate liquidity and capital preservation. The amortization or accretion of the Company’s investments is recognized as interest income in the consolidated statements of operations and was approximately $1.6 million and $0.7 million for the years ended December 31, 2023 and 2022, respectively. No allowance for credit losses on held-to-maturity securities was required as of December 31, 2023 or 2022.

The contractual maturities of the Company’s held-to-maturity investments are summarized below (in thousands):


	December 31, 2023				December 31, 2022
	Amortized Cost Basis		Aggregate Fair Value		Amortized Cost Basis		Aggregate Fair Value
Due in one year or less(1)	$	115,263	$	115,374	$	152,862	$	156,976
Due after one year through five years		7,155		7,197		1,961		1,948
Total	$	122,418	$	122,571	$	154,823	$	158,924

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The fair value of the cash equivalents, long-term and short-term investments are disclosed below (in millions):


	December 31, 2023
	Investment Level	Amortized Cost (Carrying Value)			Gross Unrealized Holding Gains		Estimated Fair Value
Commercial bonds and paper	Level 2	$	43.3		$	0.1	$	43.4
Money market funds	Level 1		69.6			—		69.6
U.S. Treasuries	Level 2		9.5			0.1		9.6
Total cash equivalents and investments			122.4			0.2		122.6
Less: cash equivalents			(58.6	)		—		(58.6	)
Total Investments		$	63.8		$	0.2	$	64.0


	December 31, 2022
	Investment Level	Amortized Cost (Carrying Value)			Gross Unrealized Holding Losses			Estimated Fair Value
Commercial bonds and paper	Level 2	$	66.8		$	(0.6	)	$	66.2
Money market funds	Level 1		51.8			—			51.8
U.S. Treasuries	Level 2		36.3			(0.6	)		35.7
Total cash equivalents and investments			154.9			(1.2	)		153.7
Less: cash equivalents			(51.8	)		—			(51.8	)
Total Investments		$	103.1		$	(1.2	)	$	101.9

As of December 31, 2023, there was $0.5 million of accrued interest receivable included in prepaid expense and other current assets in our consolidated balance sheets. As of December 31, 2022, there was $0.6 million of accrued interest receivable and $5.1 million of amount receivable on matured investment.