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Marketable Securities (Schedule of Carrying Values of Marketable Securities) (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | $ 15.9 | $ 22.3
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Estimated fair value, current | 8.5 | 17.9
Estimated fair value, noncurrent | 7.4 | 4.4
Estimated Fair Value | 15.9 | 22.3
Marketable securities current |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized cost current | 8.5 | 17.9
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Estimated fair value, current | 8.5 | 17.9
U.S. Government bonds |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized cost current | 5.9 | 8.8
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Estimated fair value, current | 5.9 | 8.8
U.S. Corporate bonds |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized cost current | 2.5 | 6.0
Amortized cost, noncurrent | 5.9 | 4.4
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Estimated fair value, current | 2.5 | 6.0
Estimated fair value, noncurrent | 5.9 | 4.4
Non U.S. Corporate bonds |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized cost current |  | 3.0
Amortized cost, noncurrent | 1.5 |
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Estimated fair value, current |  | 3.0
Estimated fair value, noncurrent | 1.5 |
Investment income receivable |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized cost current | 0.1 | 0.1
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Estimated fair value, current | 0.1 | $ 0.1
Marketable securities non-current |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized cost, noncurrent | 7.4 |
Gross Unrealized Gains | 0.0 |
Gross Unrealized Losses | 0.0 |
Estimated fair value, noncurrent | $ 7.4 |

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