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Investments (Tables)
 3 Months Ended
Mar. 31, 2024
Equity Method Investments and Joint Ventures [Abstract]  
Debt securities, held-to-maturity HTM debt securities are classified as short-term or long-term based upon the contractual maturity of the underlying investment.
March 31, 2024
(in thousands)
Amortized CostGross Unrealized GainsGross Unrealized LossesEstimated Fair Value
Accrued Interest
Short-term
US Treasury bills and notes
10,661 0$(1)$10,660 $45 
Corporate debt securities
21,158 4 (25)21,137 311 
Yankee debt securities3,000 — — 3,000 87 
Total debt securities due within a year
$34,819 $4 $(26)$34,797 $443 
Total HTM Debt Securities
34,819 4 (26)34,797 443 

December 31, 2023
(in thousands)
Amortized CostGross Unrealized GainsGross Unrealized LossesEstimated Fair Value
Accrued Interest
Short-term
US Treasury bills and notes
20,423 6$— $20,429 $14 
Corporate debt securities
21,736 14 (33)21,717 209 
Yankee debt securities3,000 — (8)2,992 43 
Total debt securities due within a year
$45,159 $20 $(41)$45,138 $266 
Total HTM Debt Securities
45,159 20 (41)45,138 266 
Investments
The Company’s equity investments consisted of the following (in thousands):
As of
March 31, 2024December 31, 2023
Equity Method Investment in LanzaJet$6,354 $7,066 
Equity Security Investment in SGLT14,990 14,990 
Total Investment
$21,344 $22,056