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INVESTMENTS - Schedule of Gross Realized Gains and Losses (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Gain (Loss) on Securities [Line Items]    
Less than 12 months, fair value $ 28.4 $ 51.1
Less than 12 months, unrealized losses (0.1) (0.7)
12 months or longer, fair value 10.4 102.5
12 months or longer, unrealized losses (0.1) (1.6)
Total fair value 38.9 153.6
Total unrealized losses (0.2) (2.2)
Short-term investments    
Gain (Loss) on Securities [Line Items]    
Less than 12 months, fair value 2.6 0.0
Less than 12 months, unrealized losses 0.0 0.0
12 months or longer, fair value 1.1 6.0
12 months or longer, unrealized losses 0.0 (0.1)
Total fair value 3.7 6.0
Total unrealized losses 0.0 (0.1)
U.S. Treasury    
Gain (Loss) on Securities [Line Items]    
Less than 12 months, fair value 4.8 3.4
Less than 12 months, unrealized losses 0.0 (0.1)
12 months or longer, fair value 1.7 5.0
12 months or longer, unrealized losses 0.0 (0.1)
Total fair value 6.5 8.5
Total unrealized losses 0.0 (0.1)
Municipal    
Gain (Loss) on Securities [Line Items]    
Less than 12 months, fair value 0.2 6.4
Less than 12 months, unrealized losses 0.0 (0.1)
12 months or longer, fair value 0.0 10.4
12 months or longer, unrealized losses 0.0 (0.1)
Total fair value 0.2 16.8
Total unrealized losses 0.0 (0.2)
Corporate    
Gain (Loss) on Securities [Line Items]    
Less than 12 months, fair value 3.3 11.4
Less than 12 months, unrealized losses 0.0 (0.1)
12 months or longer, fair value 4.6 48.0
12 months or longer, unrealized losses 0.0 (0.8)
Total fair value 8.0 59.4
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 17.4 29.8
Less than 12 months, unrealized losses (0.1) (0.4)
12 months or longer, fair value 3.0 33.1
12 months or longer, unrealized losses 0.0 (0.5)
Total fair value 20.4 62.9
Total unrealized losses $ (0.1) $ (0.9)