Note 3 - Cash, Cash Equivalents, and Short-term Investments Cash, cash equivalents, and short-term investments consisted of the following as of March 31, 2024: | | | | | | | | | | | | | | | | | | | | | | | | | (in millions) | Amortized Cost | | Unrealized Gains | | Unrealized Losses | | Estimated Fair Value | | | | | | | | | | Cash | $ | 153.0 | | | $ | — | | | $ | — | | | $ | 153.0 | | | Cash equivalents: | | | | | | | | | | | | | | | | | Money market mutual funds | 252.9 | | | — | | | — | | | 252.9 | | | | | | | | | | | | | | | | | | | Total cash and cash equivalents | $ | 405.9 | | | $ | — | | | $ | — | | | $ | 405.9 | | | Short-term investments: | | | | | | | | | Corporate debt securities | $ | 18.4 | | | $ | — | | | $ | — | | | $ | 18.4 | | | Securities guaranteed by U.S. government | 4.7 | | | — | | | — | | | 4.7 | | | Certificates of deposit | 11.2 | | | — | | | — | | | 11.2 | | | Total short-term investments | $ | 34.3 | | | $ | — | | | $ | — | | | $ | 34.3 | | | Total cash, cash equivalents, and short-term investments | $ | 440.2 | | | $ | — | | | $ | — | | | $ | 440.2 | |
Cash, cash equivalents, and short-term investments consisted of the following as of December 31, 2023: | | | | | | | | | | | | | | | | | | | | | | | | | (in millions) | Amortized Cost | | Unrealized Gains | | Unrealized Losses | | Estimated Fair Value | | | | | | | | | | Cash | $ | 201.9 | | | $ | — | | | $ | — | | | $ | 201.9 | | | Cash equivalents: | | | | | | | | | | | | | | | | | Money market mutual funds | 245.2 | | | — | | | — | | | 245.2 | | | | | | | | | | | | | | | | | | | Total cash and cash equivalents | $ | 447.1 | | | $ | — | | | $ | — | | | $ | 447.1 | | | Short-term investments: | | | | | | | | | Corporate debt securities | $ | 37.4 | | | $ | — | | | $ | — | | | $ | 37.4 | | | Securities guaranteed by U.S. government | 26.7 | | | — | | | — | | | 26.7 | | | Other government securities | 18.1 | | | — | | | — | | | 18.1 | | | Total short-term investments | $ | 82.2 | | | $ | — | | | $ | — | | | $ | 82.2 | | | Total cash, cash equivalents, and short-term investments | $ | 529.3 | | | $ | — | | | $ | — | | | $ | 529.3 | |
Refer to Note 8 - Fair Value for further information regarding the fair value of our financial instruments. Gross unrealized losses on our available-for sale securities were immaterial at March 31, 2024 and December 31, 2023. The following table summarizes the cost and estimated fair value of the securities classified as short-term investments based on stated effective maturities as of March 31, 2024 and December 31, 2023: | | | | | | | | | | | | | | | | | | | | | | | | | | | | | March 31, 2024 | | December 31, 2023 | | (in millions) | | Amortized Cost | | Estimated Fair Value | | Amortized Cost | | Estimated Fair Value | | Due within one year | | $ | 34.3 | | | $ | 34.3 | | | $ | 82.2 | | | $ | 82.2 | | | Total | | $ | 34.3 | | | $ | 34.3 | | | $ | 82.2 | | | $ | 82.2 | |
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