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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
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Segment Reporting Information [Line Items]        
Gross written premiums $ 4,609.3 $ 3,967.6 $ 4,338.7  
Net written premiums 2,942.6 2,581.9 2,896.0  
Gross earned premiums 4,387.8 4,006.8 3,988.0  
Net earned premiums 2,889.7 2,614.5 2,688.7  
Losses and loss adjustment expenses 1,717.8 1,553.0 1,680.0  
Acquisition costs 420.2 380.2 431.8  
General and administrative expenses 405.9 354.5 386.5  
Underwriting income (loss) 345.8 326.8 190.4  
Corporate and other expenses (97.3) (114.0) (71.7)  
Non-operating expenses (2) (29.9) (35.1) (36.0)  
Net investment income (1) 318.0 [1] 275.7 188.1  
Gain (loss) on investments 52.6 75.9 5.0  
Realized and unrealized investment losses [2] (102.1) (61.4) (182.6)  
Change in fair value of derivatives (21.1) 26.1 (80.5)  
Interest Expense, Debt (62.1) (55.2) (43.7)  
Net realized and unrealized foreign exchange gains 60.2 (36.2) 15.9  
Other income 0.0 0.0 8.2  
Other expenses 0.0 0.0 20.1  
Income (loss) from operations before income taxes 464.1 402.6 (27.0)  
Income Tax Expense (Benefit) 22.0 132.1 78.1  
Net income/(loss) 486.1 534.7 51.1  
Reserve for losses and loss adjustment expenses 8,122.6 7,810.6 7,710.9 $ 7,611.8
Net reserve for losses and LAE at the start of the year $ 3,950.6 $ 3,232.8 $ 2,813.2 $ 4,313.7
Ratios        
Loss ratio 59.40% 59.40% 62.50%  
Acquisition cost ratio 14.50% 14.50% 16.10%  
General and administrative expense ratio 14.00% 13.60% 14.40%  
Expense ratio 28.50% 28.10% 30.50%  
Combined ratio 87.90% 87.50% 93.00%  
Reinsurance        
Segment Reporting Information [Line Items]        
Net written premiums $ 1,275.7 $ 1,098.0 $ 1,426.4  
Net earned premiums 1,305.7 1,154.5 1,251.8  
Reserve for losses and loss adjustment expenses 1,691.5 1,373.1 1,360.7  
Net reserve for losses and LAE at the start of the year $ 1,691.5 $ 1,373.1 $ 1,360.7  
Ratios        
Loss ratio 56.80% 52.90% 61.50%  
Acquisition cost ratio 17.40% 18.10% 20.20%  
General and administrative expense ratio 10.90% 10.40% 11.40%  
Expense ratio 28.30% 28.50% 31.60%  
Combined ratio 85.10% 81.40% 93.10%  
Insurance        
Segment Reporting Information [Line Items]        
Net written premiums $ 1,666.9 $ 1,483.9 $ 1,469.6  
Net earned premiums 1,584.0 1,460.0 1,436.9  
Reserve for losses and loss adjustment expenses 2,259.1 1,859.7 1,452.5  
Net reserve for losses and LAE at the start of the year $ 2,259.1 $ 1,859.7 $ 1,452.5  
Ratios        
Loss ratio 61.60% 64.50% 63.30%  
Acquisition cost ratio 12.20% 11.80% 12.50%  
General and administrative expense ratio 16.70% 16.00% 17.00%  
Expense ratio 28.90% 27.80% 29.50%  
Combined ratio 90.50% 92.30% 92.80%  
Operating Segments | Reinsurance        
Segment Reporting Information [Line Items]        
Gross written premiums $ 1,885.8 $ 1,521.0 $ 1,807.0  
Net written premiums 1,275.7 1,098.0 1,426.4  
Gross earned premiums 1,822.1 1,562.0 1,617.2  
Net earned premiums 1,305.7 1,154.5 1,251.8  
Losses and loss adjustment expenses 741.3 611.1 770.3  
Acquisition costs 227.0 208.6 252.4  
General and administrative expenses 141.7 120.6 142.5  
Underwriting income (loss) 195.7 214.2 86.6  
Operating Segments | Insurance        
Segment Reporting Information [Line Items]        
Gross written premiums 2,723.5 2,446.6 2,531.7  
Net written premiums 1,666.9 1,483.9 1,469.6  
Gross earned premiums 2,565.7 2,444.8 2,370.8  
Net earned premiums 1,584.0 1,460.0 1,436.9  
Losses and loss adjustment expenses 976.5 941.9 909.7  
Acquisition costs 193.2 171.6 179.4  
General and administrative expenses 264.2 233.9 244.0  
Underwriting income (loss) $ 150.1 $ 112.6 $ 103.8  
[1] Net investment income includes related party net investment income of $15.2 million (2023 — $19.6 million, 2022 — $3.1 million) and related party investment management fees of $9.2 million (2023 — $9.4 million, 2022 — $4.9 million).
[2] Realized and unrealized investments gains includes gains of $2.5 million on related party investments (2023 — gains of $8.7 million, 2022 — loss of $0.4 million).