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INVESTMENTS - Schedule of Gross Realized Gains and Losses (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Gain (Loss) on Securities [Line Items] |  |
Less than 12 months, fair value | $ 45.4 | $ 147.5
Less than 12 months, unrealized losses | (0.3) | (1.6)
12 months or longer, fair value | 13.1 | 59.8
12 months or longer, unrealized losses | (0.2) | (1.2)
Total fair value | 58.6 | 207.3
Total unrealized losses | (0.5) | (2.7)
Short-term investments |  |
Gain (Loss) on Securities [Line Items] |  |
Less than 12 months, fair value | 0.0 | 0.3
Less than 12 months, unrealized losses | 0.0 | 0.0
12 months or longer, fair value | 0.8 | 3.9
12 months or longer, unrealized losses | 0.0 | 0.0
Total fair value | 0.8 | 4.1
Total unrealized losses | 0.0 | 0.0
U.S. Treasury |  |
Gain (Loss) on Securities [Line Items] |  |
Less than 12 months, fair value | 0.8 | 1.1
Less than 12 months, unrealized losses | 0.0 | 0.0
12 months or longer, fair value | 0.8 | 1.4
12 months or longer, unrealized losses | 0.0 | 0.0
Total fair value | 1.7 | 2.5
Total unrealized losses | 0.0 | 0.0
Municipal |  |
Gain (Loss) on Securities [Line Items] |  |
Less than 12 months, fair value | 0.0 | 3.4
Less than 12 months, unrealized losses | 0.0 | (0.1)
12 months or longer, fair value | 0.0 | 1.6
12 months or longer, unrealized losses | 0.0 | 0.0
Total fair value | 0.0 | 4.9
Total unrealized losses | 0.0 | (0.1)
Corporate |  |
Gain (Loss) on Securities [Line Items] |  |
Less than 12 months, fair value | 4.9 | 64.3
Less than 12 months, unrealized losses | 0.0 | (0.5)
12 months or longer, fair value | 0.0 | 26.6
12 months or longer, unrealized losses | 0.0 | (0.4)
Total fair value | 4.9 | 90.9
Total unrealized losses | 0.0 | (0.9)
Mortgage and other asset-backed securities |  |
Gain (Loss) on Securities [Line Items] |  |
Less than 12 months, fair value | 39.7 | 78.5
Less than 12 months, unrealized losses | (0.3) | (1.0)
12 months or longer, fair value | 11.5 | 26.4
12 months or longer, unrealized losses | (0.2) | (0.7)
Total fair value | 51.2 | 104.9
Total unrealized losses | $ (0.5) | $ (1.7)

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