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FAIR VALUE MEASUREMENTS - Fair Value of Other Temporary Investments and Cash (Details) - USD ($) $ in Millions | Jun. 30, 2025 | Dec. 31, 2024
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Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Fair Value | $ 4,609.7 | $ 4,395.1
Total Other Temporary Investments and Restricted Cash |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cost | 241.2 | 236.9
Gross Unrealized Gains | 25.1 | 26.9
Gross Unrealized Losses | (3.0) | (5.3)
Fair Value | 263.3 | 258.5
Restricted Cash |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cost | 51.3 | 43.1
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Value | 51.3 | 43.1
Other Cash Deposits |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cost | 16.1 | 13.9
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair Value | 16.1 | 13.9
Fixed Income Securities – Mutual Funds |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cost | 159.8 | 167.2
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (3.0) | (5.3)
Fair Value | 156.8 | 161.9
Equity Securities – Mutual Funds |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cost | 14.0 | 12.7
Gross Unrealized Gains | 25.1 | 26.9
Gross Unrealized Losses | 0.0 | 0.0
Fair Value | $ 39.1 | $ 39.6

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