| 2023 | 2022 | 2021 | |||||||||||||||||||||||||||||
| Balance at beginning of year | $ | 1.1 | $ | 2.2 | $ | 0.4 | Expected credit losses on securities where credit losses were not previously recognized | 4.1 | — | 1.8 | Reductions for expected credit losses on securities where credit losses were previously recognized | (3.5) | (1.1) | — | Securities sold/redeemed/matured | (0.4) | — | — | Balance at end of year | $ | 1.3 | $ | 1.1 | $ | 2.2 | ||||||
| December 31, 2023 | ||||||||||||||||||||
| Amortized Cost | Unrealized gains | Unrealized losses | Fair value | |||||||||||||||||
| U.S. Treasuries | $ | 49.0 | $ | — | $ | — | $ | 49.0 | Total short-term investments | $ | 49.0 | $ | — | $ | — | $ | 49.0 | |||
