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Fair Value Measurements and Investments Available-for-Sale Investments (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | $ 1,136.0 |
Debt Securities, Available-for-sale, Amortized Cost | [1] | 1,142.2 |
Fair Value, Recurring [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 1,136.0 | $ 1,300.0
Fair Value, Recurring [Member] | Corporate Debt Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 531.6 | 699.3
Fair Value, Recurring [Member] | Municipal Obligations [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 12.1 | 23.1
Fair Value, Recurring [Member] | Asset-backed Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 323.7 | 333.4
Fair Value, Recurring [Member] | US Government Sponsored Agencies [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 255.9 | 230.7
Fair Value, Recurring [Member] | Foreign Government Obligations [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 12.7 | 13.5
Short-term Investments [Member] | Fair Value, Recurring [Member] | Corporate Debt Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 531.6 | 699.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | [1] | (3.3) | (10.8)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | [1] | 0.8 | 0.2
Debt Securities, Available-for-sale, Allowance for Credit Loss | [1] | 0.0 |
Debt Securities, Available-for-sale, Amortized Cost | [1] | 534.1 | 709.9
Short-term Investments [Member] | Fair Value, Recurring [Member] | Municipal Obligations [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 12.1 | 23.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | [1] | (0.1) | (0.3)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | [1] | 0.0 | 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss | [1] | 0.0 |
Debt Securities, Available-for-sale, Amortized Cost | [1] | 12.2 | 23.4
Short-term Investments [Member] | Fair Value, Recurring [Member] | Asset-backed Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 323.7 | 333.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | [1] | (2.7) | (6.3)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | [1] | 0.7 | 0.1
Debt Securities, Available-for-sale, Allowance for Credit Loss | [1] | 0.0 |
Debt Securities, Available-for-sale, Amortized Cost | [1] | 325.7 | 339.6
Short-term Investments [Member] | Fair Value, Recurring [Member] | US Government Sponsored Agencies [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 255.9 | 230.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | [1] | (1.6) | (3.2)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | [1] | 0.1 | 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss | [1] | 0.0 |
Debt Securities, Available-for-sale, Amortized Cost | [1] | 257.4 | 233.9
Short-term Investments [Member] | Fair Value, Recurring [Member] | Foreign Government Obligations [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 12.7 | 13.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | [1] | (0.1) | (0.3)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | [1] | 0.0 | 0.0
Debt Securities, Available-for-sale, Allowance for Credit Loss | [1] | 0.0 |
Debt Securities, Available-for-sale, Amortized Cost | [1] | 12.8 | 13.8
Short-term Investments [Member] | Fair Value, Recurring [Member] | Debt Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt Securities, Available-for-sale | [1] | 1,136.0 | 1,300.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | [1] | (7.8) | (20.9)
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | [1] | 1.6 | 0.3
Debt Securities, Available-for-sale, Allowance for Credit Loss | [1] | 0.0 |
Debt Securities, Available-for-sale, Amortized Cost | [1] | $ 1,142.2 | $ 1,320.6
[1] | Available-for-sale investments are included in Short-term investments in the consolidated balance sheets.

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