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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, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Segment Reporting Information [Line Items]        
Gross written premiums $ 3,967.6 $ 4,338.7 $ 3,938.4  
Net written premiums 2,581.9 2,896.0 2,587.7  
Gross earned premiums 4,006.8 3,988.0 3,618.3  
Net earned premiums 2,614.5 2,688.7 2,410.5  
Losses and loss adjustment expenses 1,553.0 1,680.0 1,693.3  
Acquisition costs 380.2 431.8 414.1  
General and administrative expenses 354.5 386.5 333.1  
Underwriting income (loss) 326.8 190.4 (30.0)  
Corporate Expenses (114.0) (71.7) (64.3)  
Non-operating expenses (1) (35.1) (36.0) (20.6)  
Net investment income (1) 275.7 188.1 147.5  
Realized and unrealized investment gains 75.9 5.0 56.2  
Realized and unrealized investment losses (61.4) (182.6) (47.4)  
Change in fair value of derivatives 26.1 (80.5) (35.9)  
Interest Expense, Debt (55.2) (43.7) (14.3)  
Net realized and unrealized foreign exchange gains (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 402.6 (27.0) 35.1  
Income Tax Expense (Benefit) 132.1 78.1 (5.3)  
Net income/(loss) 534.7 51.1 29.8  
Losses and loss adjustment expenses 7,810.6 7,710.9 7,611.8 $ 7,165.3
Liability for Unpaid Claims and Claims Adjustment Expense, Net, Total $ 3,232.8 $ 2,813.2 $ 4,313.7 $ 3,970.1
Ratios        
Loss ratio 59.40% 62.50% 70.20%  
Acquisition cost ratio 14.50% 16.10% 17.20%  
General and administrative expense ratio 13.60% 14.40% 13.80%  
Expense ratio 28.10% 30.50% 31.00%  
Combined ratio 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 1,373.1 1,360.7 2,148.4  
Liability for Unpaid Claims and Claims Adjustment Expense, Net, Total $ 1,373.1 $ 1,360.7 $ 2,148.4  
Ratios        
Loss ratio 52.90% 61.50% 63.00%  
Acquisition cost ratio 18.10% 20.20% 19.80%  
General and administrative expense ratio 10.40% 11.40% 10.80%  
Expense ratio 28.50% 31.60% 30.60%  
Combined ratio 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 1,859.7 1,452.5 2,165.3  
Liability for Unpaid Claims and Claims Adjustment Expense, Net, Total $ 1,859.7 $ 1,452.5 $ 2,165.3  
Ratios        
Loss ratio 64.50% 63.30% 76.50%  
Acquisition cost ratio 11.80% 12.50% 14.90%  
General and administrative expense ratio 16.00% 17.00% 16.40%  
Expense ratio 27.80% 29.50% 31.30%  
Combined ratio 92.30% 92.80% 107.80%  
Operating Segments | Reinsurance        
Segment Reporting Information [Line Items]        
Gross written premiums $ 1,521.0 $ 1,807.0 $ 1,597.0  
Net written premiums 1,098.0 1,426.4 1,199.0  
Gross earned premiums 1,562.0 1,617.2 1,479.2  
Net earned premiums 1,154.5 1,251.8 1,118.8  
Losses and loss adjustment expenses 611.1 770.3 705.2  
Acquisition costs 208.6 252.4 221.6  
General and administrative expenses 120.6 142.5 121.3  
Underwriting income (loss) 214.2 86.6 70.7  
Operating Segments | Insurance        
Segment Reporting Information [Line Items]        
Gross written premiums 2,446.6 2,531.7 2,341.4  
Net written premiums 1,483.9 1,469.6 1,388.7  
Gross earned premiums 2,444.8 2,370.8 2,139.1  
Net earned premiums 1,460.0 1,436.9 1,291.7  
Losses and loss adjustment expenses 941.9 909.7 988.1  
Acquisition costs 171.6 179.4 192.5  
General and administrative expenses 233.9 244.0 211.8  
Underwriting income (loss) $ 112.6 $ 103.8 $ (100.7)