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Reinsurance (Details)
$ in Millions
12 Months Ended
Dec. 31, 2023
USD ($)
item
Dec. 31, 2022
USD ($)
item
Jun. 30, 2024
USD ($)
Ceded Credit Risk [Line Items]      
Reinsurance recoverable assets $ 18,346.0 $ 13,541.3 $ 17,988.9
Reinsurance recoverable liabilities 45.2 39.4 43.3
Reinsurance deposit receivable 6,078.7 7,901.0 $ 5,383.0
Reinsurer Concentration Risk | Net ceded reinsurance recoverables | Five largest ceded reinsurers      
Ceded Credit Risk [Line Items]      
Net ceded reinsurance recoverables $ 18,093.5 $ 12,953.1  
Concentration risk percentage 99.00% 98.00%  
Number of largest ceded reinsurers | item 5 5