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Schedule III - Supplementary Insurance Information (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]                
Deferred Policy Acquisition Costs          $ 296.2 $ 319.0 $ 290.8  
Net Reserves for Losses and LAE  $ 3,770.0 $ 3,174.1 $ 3,770.0 $ 3,174.1 3,232.8 2,813.2 4,313.7 $ 3,970.1
Net Reserves for Unearned Premiums         1,692.8 1,720.2 1,516.2  
Net premiums earned 698.3 660.0 2,069.4 1,947.4 2,614.5 2,688.7 2,410.5  
Losses and LAE Expenses          1,553.0 1,680.0 1,693.3  
Policy Acquisition Expenses          380.2 431.8 414.1  
Net written premiums $ 673.6 $ 628.6 $ 2,225.6 $ 1,979.1 2,581.9 2,896.0 2,587.7  
General and Administrative Expenses          354.5 386.5 333.1  
Net Investment Income          275.7 188.1 147.5  
Reinsurance                
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]                
Deferred Policy Acquisition Costs          201.5 227.2 205.2  
Net Reserves for Losses and LAE          1,373.1 1,360.7 2,148.4  
Net Reserves for Unearned Premiums         644.5 862.4 688.6  
Net premiums earned         1,154.5 1,251.8 1,118.8  
Losses and LAE Expenses          611.1 770.3 705.2  
Policy Acquisition Expenses          208.6 252.4 221.6  
Net written premiums         1,098.0 1,426.4 1,199.0  
General and Administrative Expenses          120.6 142.5 121.3  
Insurance                
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]                
Deferred Policy Acquisition Costs          94.7 91.8 85.6  
Net Reserves for Losses and LAE          1,859.7 1,452.5 2,165.3  
Net Reserves for Unearned Premiums         1,048.3 857.8 827.6  
Net premiums earned         1,460.0 1,436.9 1,291.7  
Losses and LAE Expenses          941.9 909.7 988.1  
Policy Acquisition Expenses          171.6 179.4 192.5  
Net written premiums         1,483.9 1,469.6 1,388.7  
General and Administrative Expenses          $ 233.9 $ 244.0 $ 211.8