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Segment Reporting - Summary of Gross and Net Written and Earned Premiums, Underwriting Results, Ratios and Reserves for Each of Company's Business Segments (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
Segment Reporting Information [Line Items]                
Gross written premiums $ 1,116.8 $ 982.7 $ 3,598.6 $ 3,107.9 $ 3,967.6 $ 4,338.7 $ 3,938.4  
Net written premiums 673.6 628.6 2,225.6 1,979.1 2,581.9 2,896.0 2,587.7  
Gross earned premiums 1,102.2 1,021.2 3,174.4 2,980.5 4,006.8 3,988.0 3,618.3  
Net earned premiums 698.3 660.0 2,069.4 1,947.4 2,614.5 2,688.7 2,410.5  
Losses and loss adjustment expenses 474.2 430.5 1,278.9 1,147.6 1,553.0 1,680.0 1,693.3  
Acquisition costs 91.7 94.3 290.2 287.2 380.2 431.8 414.1  
General and administrative expenses 99.4 87.3 297.5 256.3 354.5 386.5 333.1  
Underwriting income (loss) 33.0 47.9 202.8 256.3 326.8 190.4 (30.0)  
Corporate and other expenses (18.6) (32.6) (83.3) (85.7) (114.0) (71.7) (64.3)  
Non-operating expenses (7.6) (0.2) (19.3) (10.8) (35.1) (36.0) (20.6)  
Net investment income 79.6 77.9 238.9 207.3 275.7 188.1 147.5  
Realized and unrealized investment gains (losses) 22.0 11.3 48.8 47.4 75.9 5.0 56.2  
Realized and unrealized investment losses (15.9) (13.1) (69.8) (31.2) (61.4) (182.6) (47.4)  
Change in fair value of derivatives 37.8 (35.6) 22.2 (16.0) 26.1 (80.5) (35.9)  
Interest expense         (55.2) (43.7) (14.3)  
Net realized and unrealized foreign exchange gains (46.3) 34.1 (19.7) 20.4 (36.2) 15.9 40.0  
Other income         0.0 8.2 14.7  
Other expenses         0.0 20.1 10.8  
Income (loss) from operations before income taxes 63.1 91.5 269.6 346.6 402.6 (27.0) 35.1  
Income Tax Expense (Benefit) (6.4) (5.2) (32.1) (41.4) 132.1 78.1 (5.3)  
Net income/(loss) 56.7 86.3 237.5 305.2 534.7 51.1 29.8  
Losses and loss adjustment expenses 8,201.3 7,770.8 8,201.3 7,770.8 7,810.6 7,710.9 7,611.8 $ 7,165.3
Net reserves for losses and loss adjustment expenses $ 3,770.0 $ 3,174.1 $ 3,770.0 $ 3,174.1 $ 3,232.8 $ 2,813.2 $ 4,313.7 $ 3,970.1
Ratios                
Loss ratio 67.90% 65.20% 61.80% 58.90% 59.40% 62.50% 70.20%  
Acquisition cost ratio 13.10% 14.30% 14.00% 14.70% 14.50% 16.10% 17.20%  
General and administrative expense ratio 14.20% 13.20% 14.40% 13.20% 13.60% 14.40% 13.80%  
Expense ratio 27.30% 27.50% 28.40% 27.90% 28.10% 30.50% 31.00%  
Combined ratio 95.20% 92.70% 90.20% 86.80% 87.50% 93.00% 101.20%  
Reinsurance                
Segment Reporting Information [Line Items]                
Net written premiums         $ 1,098.0 $ 1,426.4 $ 1,199.0  
Net earned premiums         1,154.5 1,251.8 1,118.8  
Losses and loss adjustment expenses $ 1,655.0 $ 1,404.8 $ 1,655.0 $ 1,404.8 1,373.1 1,360.7 2,148.4  
Net reserves for losses and loss adjustment expenses         $ 1,373.1 $ 1,360.7 $ 2,148.4  
Ratios                
Loss ratio 71.10% 53.00% 59.80% 52.30% 52.90% 61.50% 63.00%  
Acquisition cost ratio 15.00% 18.80% 17.10% 18.60% 18.10% 20.20% 19.80%  
General and administrative expense ratio 14.00% 10.20% 12.30% 10.20% 10.40% 11.40% 10.80%  
Expense ratio 29.00% 29.00% 29.40% 28.80% 28.50% 31.60% 30.60%  
Combined ratio 100.10% 82.00% 89.20% 81.10% 81.40% 93.10% 93.60%  
Insurance                
Segment Reporting Information [Line Items]                
Net written premiums         $ 1,483.9 $ 1,469.6 $ 1,388.7  
Net earned premiums         1,460.0 1,436.9 1,291.7  
Losses and loss adjustment expenses $ 2,115.0 $ 1,769.3 $ 2,115.0 $ 1,769.3 1,859.7 1,452.5 2,165.3  
Net reserves for losses and loss adjustment expenses         $ 1,859.7 $ 1,452.5 $ 2,165.3  
Ratios                
Loss ratio 65.60% 74.90% 63.40% 64.20% 64.50% 63.30% 76.50%  
Acquisition cost ratio 11.80% 10.70% 11.50% 11.70% 11.80% 12.50% 14.90%  
General and administrative expense ratio 14.40% 15.60% 16.10% 15.50% 16.00% 17.00% 16.40%  
Expense ratio 26.20% 26.30% 27.60% 27.20% 27.80% 29.50% 31.30%  
Combined ratio 91.80% 101.20% 91.00% 91.40% 92.30% 92.80% 107.80%  
Operating Segments | Reinsurance                
Segment Reporting Information [Line Items]                
Gross written premiums $ 416.2 $ 367.5 $ 1,596.8 $ 1,243.2 $ 1,521.0 $ 1,807.0 $ 1,597.0  
Net written premiums 263.7 261.2 1,052.0 866.2 1,098.0 1,426.4 1,199.0  
Gross earned premiums 449.7 403.1 1,298.2 1,158.8 1,562.0 1,617.2 1,479.2  
Net earned premiums 293.5 291.8 925.3 863.8 1,154.5 1,251.8 1,118.8  
Losses and loss adjustment expenses 208.6 154.7 553.0 451.9 611.1 770.3 705.2  
Acquisition costs 43.9 54.9 158.5 160.6 208.6 252.4 221.6  
General and administrative expenses 41.2 29.7 113.7 88.1 120.6 142.5 121.3  
Underwriting income (loss) (0.2) 52.5 100.1 163.2 214.2 86.6 70.7  
Operating Segments | Insurance                
Segment Reporting Information [Line Items]                
Gross written premiums 700.6 615.2 2,001.8 1,864.7 2,446.6 2,531.7 2,341.4  
Net written premiums 409.9 367.4 1,173.6 1,112.9 1,483.9 1,469.6 1,388.7  
Gross earned premiums 652.5 618.1 1,876.2 1,821.7 2,444.8 2,370.8 2,139.1  
Net earned premiums 404.8 368.2 1,144.1 1,083.6 1,460.0 1,436.9 1,291.7  
Losses and loss adjustment expenses 265.6 275.8 725.9 695.7 941.9 909.7 988.1  
Acquisition costs 47.8 39.4 131.7 126.6 171.6 179.4 192.5  
General and administrative expenses 58.2 57.6 183.8 168.2 233.9 244.0 211.8  
Underwriting income (loss) $ 33.2 $ (4.6) $ 102.7 $ 93.1 $ 112.6 $ 103.8 $ (100.7)