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Investments - Allowance for Credit Loss (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Rollforward of the allowance for credit loss by major security type    
Beginning balance $ 7.4  
Additions for credit losses not previously recorded 0.3 $ 7.6
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period 2.8  
Ending balance 10.5 7.6
Corporate debt securities    
Rollforward of the allowance for credit loss by major security type    
Beginning balance 0.9  
Additions for credit losses not previously recorded   7.0
Ending balance 0.9 7.0
Commercial mortgage-backed securities    
Rollforward of the allowance for credit loss by major security type    
Beginning balance 4.3  
Additions for credit losses not previously recorded 0.3 0.4
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period 2.4  
Ending balance 7.0 0.4
Collateralized debt obligations    
Rollforward of the allowance for credit loss by major security type    
Beginning balance 2.2  
Additions for credit losses not previously recorded   0.1
Additional increases (decreases) for credit losses on securities with an allowance recorded in the previous period 0.4  
Ending balance $ 2.6 0.1
Other debt obligations    
Rollforward of the allowance for credit loss by major security type    
Additions for credit losses not previously recorded   0.1
Ending balance   $ 0.1