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Marketable Securities (Tables) | 9 Months Ended
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Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract] |
Schedule of Carrying Values of Marketable Securities | The carrying values of marketable securities and the balance sheet location at September 30, 2023 and December 31, 2022 consisted of the following:
|  | September 30, 2023
(Millions of dollars) |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
Available-for-sale securities: |  |  |  |  |  |  |  |
Marketable securities current |  |  |  |  |  |  |  |
U.S. Government bonds |  | $ | 5.9 |  |  | $ | — |  |  | — |  |  | $ | 5.9 |
U.S. Corporate bonds |  | 2.5 |  |  | — |  |  | — |  |  | 2.5 |
Investment income receivable |  | 0.1 |  |  | — |  |  | — |  |  | 0.1 |
|  | 8.5 |  |  | — |  |  | — |  |  | 8.5 |
|  | September 30, 2023
(Millions of dollars) |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
Marketable securities non-current |  |  |  |  |  |  |  |
U.S. Corporate bonds |  | 5.9 |  |  | — |  |  | — |  |  | 5.9 |
Non U.S. Corporate bonds |  | 1.5 |  |  | — |  |  | — |  |  | 1.5 |
|  | 7.4 |  |  | — |  |  | — |  |  | 7.4 |
Total marketable securities |  | $ | 15.9 |  |  | $ | — |  |  | $ | — |  |  | $ | 15.9 |
|  | December 31, 2022
(Millions of dollars) |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
Available-for-sale securities: |  |  |  |  |  |  |  |
Marketable securities current |  |  |  |  |  |  |  |
U.S. Government bonds |  | $ | 8.8 |  |  | $ | — |  |  | $ | — |  |  | $ | 8.8 |
U.S. Corporate bonds |  | 6.0 |  |  | — |  |  | — |  |  | 6.0 |
Non U.S. Corporate bonds |  | 3.0 |  |  | — |  |  | — |  |  | 3.0 |
Investment income receivable |  | 0.1 |  |  | — |  |  | — |  |  | 0.1 |
|  | 17.9 |  |  | — |  |  | — |  |  | 17.9 |
Marketable securities non-current |  |  |  |  |  |  |  |
U.S. Corporate bonds |  | 4.4 |  |  | — |  |  | — |  |  | 4.4 |
Total marketable securities |  | $ | 22.3 |  |  | $ | — |  |  | $ | — |  |  | $ | 22.3 |
Schedule of Amortized Cost Basis and Fair Value of Available-for-Sale Marketable Securities | The amortized cost basis and fair value of the Company's available-for-sale marketable securities at September 30, 2023, by contractual maturity, are as follows:
(Millions of dollars) |  | Amortized Cost |  | Fair Value
Less than 1 year |  | $ | 15.8 |  |  | $ | 15.8 |
1 to 2 years |  | — |  |  | — |
Total |  | $ | 15.8 |  |  | $ | 15.8 |

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