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Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses, Allowance for Credit Losses and Fair Value of Debt Securities Available-For-Sale (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
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Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost, current | $ 781.7 | $ 735.8
Unrealized gain, current | 0.3 | 0.0
Unrealized loss, current | (1.5) | (9.4)
Fair value, current | 780.5 | 726.4
Amortized cost, noncurrent | 684.6 | 304.5
Unrealized gain, noncurrent | 3.4 | 0.2
Unrealized loss, noncurrent | (0.5) | (5.3)
Fair value, noncurrent | 687.5 | 299.4
Commercial paper |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost, current | 53.5 | 156.2
Unrealized gain, current | 0.0 | 0.0
Unrealized loss, current | 0.0 | (0.2)
Fair value, current | 53.5 | 156.0
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost, current | 382.1 | 296.2
Unrealized gain, current | 0.1 | 0.0
Unrealized loss, current | (1.0) | (3.2)
Fair value, current | 381.2 | 293.0
Amortized cost, noncurrent | 483.5 | 259.5
Unrealized gain, noncurrent | 2.9 | 0.2
Unrealized loss, noncurrent | (0.4) | (4.3)
Fair value, noncurrent | 486.0 | 255.4
Securities of government-sponsored entities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost, current | 346.1 | 283.4
Unrealized gain, current | 0.2 | 0.0
Unrealized loss, current | (0.5) | (6.0)
Fair value, current | 345.8 | 277.4
Amortized cost, noncurrent | 201.1 | 45.0
Unrealized gain, noncurrent | 0.5 | 0.0
Unrealized loss, noncurrent | (0.1) | (1.0)
Fair value, noncurrent | $ 201.5 | $ 44.0

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