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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 | Mar. 31, 2024 | Dec. 31, 2023
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Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost, current | $ 815.7 | $ 781.7
Unrealized gain, current | 0.1 | 0.3
Unrealized loss, current | (1.5) | (1.5)
Fair value, current | 814.3 | 780.5
Amortized cost, noncurrent | 701.4 | 684.6
Unrealized gain, noncurrent | 0.9 | 3.4
Unrealized loss, noncurrent | (1.9) | (0.5)
Fair value, noncurrent | 700.4 | 687.5
Commercial paper |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost, current | 58.0 | 53.5
Unrealized gain, current | 0.0 | 0.0
Unrealized loss, current | 0.0 | 0.0
Fair value, current | 58.0 | 53.5
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost, current | 419.4 | 382.1
Unrealized gain, current | 0.1 | 0.1
Unrealized loss, current | (0.9) | (1.0)
Fair value, current | 418.6 | 381.2
Amortized cost, noncurrent | 530.0 | 483.5
Unrealized gain, noncurrent | 0.9 | 2.9
Unrealized loss, noncurrent | (1.3) | (0.4)
Fair value, noncurrent | 529.6 | 486.0
Securities of government-sponsored entities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost, current | 338.3 | 346.1
Unrealized gain, current | 0.0 | 0.2
Unrealized loss, current | (0.6) | (0.5)
Fair value, current | 337.7 | 345.8
Amortized cost, noncurrent | 171.4 | 201.1
Unrealized gain, noncurrent | 0.0 | 0.5
Unrealized loss, noncurrent | (0.6) | (0.1)
Fair value, noncurrent | $ 170.8 | $ 201.5

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