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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)