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Investments - Aging Of Gross Unrealized Losses (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Fair Value      
Less than 12 Months $ 608.7 $ 0.0 $ 0.2
12 Months or More 0.0 0.0 2.2
Fair Value 608.7 0.0 2.4
Gross Unrealized Losses      
Less than 12 Months (1.1) 0.0 0.0
12 Months or More 0.0 0.0 0.0
Gross Unrealized Losses (1.1) 0.0 0.0
U.S. Government obligations      
Fair Value      
Less than 12 Months 103.9 0.0 0.2
12 Months or More 0.0 0.0 2.2
Fair Value 103.9 0.0 2.4
Gross Unrealized Losses      
Less than 12 Months (0.2) 0.0 0.0
12 Months or More 0.0 0.0 0.0
Gross Unrealized Losses (0.2) 0.0 $ 0.0
Corporate debt securities      
Fair Value      
Less than 12 Months 504.8 0.0  
12 Months or More 0.0 0.0  
Fair Value 504.8 0.0  
Gross Unrealized Losses      
Less than 12 Months (0.9) 0.0  
12 Months or More 0.0 0.0  
Gross Unrealized Losses (0.9) 0.0  
Municipal securities      
Fair Value      
Less than 12 Months 0.0 0.0  
12 Months or More 0.0 0.0  
Fair Value 0.0 0.0  
Gross Unrealized Losses      
Less than 12 Months 0.0 0.0  
12 Months or More 0.0 0.0  
Gross Unrealized Losses $ 0.0 $ 0.0