v3.26.1
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, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Segment Reporting Information [Line Items]        
Gross written premiums $ 4,673.2 $ 4,609.3 $ 3,967.6  
Net written premiums 2,836.0 2,942.6 2,581.9  
Gross earned premiums 4,602.2 4,387.8 4,006.8  
Net earned premiums [1] 2,831.9 2,889.7 2,614.5  
Losses and loss adjustment expenses [1] (1,546.7) (1,717.8) (1,553.0)  
Acquisition costs (422.4) (420.2) (380.2)  
General and administrative expenses (492.0) (405.9) (354.5)  
Underwriting income (loss) 370.8 345.8 326.8  
Corporate and other expenses (102.6) (97.3) (114.0)  
Non-operating expenses (1) (76.8) (29.9) (35.1)  
Net Investment Income 326.3 [2] 318.0 275.7  
Gain (loss) on investments 61.4 52.6 75.9  
Realized and unrealized investment losses (3) [3] (72.3) (102.1) (61.4)  
Change in fair value of derivatives 47.5 (21.1) 26.1  
Interest expense (31.7) (62.1) (55.2)  
Net realized and unrealized foreign exchange losses (78.5) 60.2 (36.2)  
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest 444.1 464.1 402.6  
Income tax (expense)/benefit (103.9) 22.0 132.1  
Net income/(loss) 340.2 486.1 534.7  
Liability for Claims and Claims Adjustment Expense 8,725.0 [4] 8,122.6 [4] 7,810.6 $ 7,710.9
Net reserve for losses and loss adjustment expenses at the start of the year $ 4,443.1 $ 3,950.6 $ 3,232.8 $ 2,813.2
Ratios        
Loss ratio 54.60% 59.40% 59.40%  
Acquisition cost ratio 14.90% 14.50% 14.50%  
General and administrative expense ratio 17.40% 14.00% 13.60%  
Expense ratio 32.30% 28.50% 28.10%  
Combined ratio 86.90% 87.90% 87.50%  
Reinsurance        
Segment Reporting Information [Line Items]        
Net written premiums $ 1,604.9 $ 1,666.9 $ 1,483.9  
Net earned premiums 1,623.8 1,584.0 1,460.0  
Liability for Claims and Claims Adjustment Expense 2,509.0 2,259.1 1,859.7  
Net reserve for losses and loss adjustment expenses at the start of the year $ 2,509.0 $ 2,259.1 $ 1,859.7  
Ratios        
Loss ratio 53.20% 61.60% 64.50%  
Acquisition cost ratio 15.00% 12.20% 11.80%  
General and administrative expense ratio 18.70% 16.70% 16.00%  
Expense ratio 33.70% 28.90% 27.80%  
Combined ratio 86.90% 90.50% 92.30%  
Insurance        
Segment Reporting Information [Line Items]        
Net written premiums $ 1,231.1 $ 1,275.7 $ 1,098.0  
Net earned premiums 1,208.1 1,305.7 1,154.5  
Liability for Claims and Claims Adjustment Expense 1,934.1 1,691.5 1,373.1  
Net reserve for losses and loss adjustment expenses at the start of the year $ 1,934.1 $ 1,691.5 $ 1,373.1  
Ratios        
Loss ratio 56.50% 56.80% 52.90%  
Acquisition cost ratio 14.80% 17.40% 18.10%  
General and administrative expense ratio 15.60% 10.90% 10.40%  
Expense ratio 30.40% 28.30% 28.50%  
Combined ratio 86.90% 85.10% 81.40%  
Operating Segments | Reinsurance        
Segment Reporting Information [Line Items]        
Gross written premiums $ 2,768.1 $ 2,723.5 $ 2,446.6  
Net written premiums 1,604.9 1,666.9 1,483.9  
Gross earned premiums 2,752.2 2,565.7 2,444.8  
Net earned premiums 1,623.8 1,584.0 1,460.0  
Losses and loss adjustment expenses (864.4) (976.5) (941.9)  
Acquisition costs (243.4) (193.2) (171.6)  
General and administrative expenses (303.3) (264.2) (233.9)  
Underwriting income (loss) 212.7 150.1 112.6  
Operating Segments | Insurance        
Segment Reporting Information [Line Items]        
Gross written premiums 1,905.1 1,885.8 1,521.0  
Net written premiums 1,231.1 1,275.7 1,098.0  
Gross earned premiums 1,850.0 1,822.1 1,562.0  
Net earned premiums 1,208.1 1,305.7 1,154.5  
Losses and loss adjustment expenses (682.3) (741.3) (611.1)  
Acquisition costs (179.0) (227.0) (208.6)  
General and administrative expenses (188.7) (141.7) (120.6)  
Underwriting income (loss) $ 158.1 $ 195.7 $ 214.2  
[1] Net earned premiums and losses and loss adjustment expenses include related party balances of $5.9 million and $3.7 million, respectively, for the twelve months ended December 31, 2025. Refer to Note 20, “Related Party Transactions” for further details.
[2] Net investment income includes related party net investment income of $13.6 million (2024 — $15.2 million, 2023 — $19.6 million) and related party investment management fees of $6.4 million (2024 — $9.2 million, 2023 — $9.4 million).
[3] Realized and unrealized investment gains and losses include losses of $1.7 million on related party investments (2024 — gains of $2.5 million, 2023 — gains of $8.7 million).
[4] Included within underwriting premiums receivables, reserve for losses and loss adjustment expenses and unearned premiums are related party balances of $1.6 million, $3.7 million and $1.2 million, respectively. Refer to Note 20, “Related Party Transactions” for further details.