v3.26.3
Segments - Schedule of Group Segment Disclosures (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting [Line Items]        
Gross premiums written $ 839.8 $ 797.5 $ 3,977.0 $ 3,739.4
Net premiums written 592.3 504.2 2,541.8 2,229.5
Net premiums earned 783.7 599.8 1,933.3 1,740.8
Losses and loss adjustment expenses (351.9) (180.7) (868.3) (906.6)
Policy acquisition expenses (335.3) (266.4) (831.0) (752.3)
General and administrative expenses (29.5) (27.2) (88.0) (71.5)
Underwriting income 67.0 125.5 146.0 10.4
Net investment income 44.6 45.9 132.3 140.0
Net realized and unrealized investment gains/(losses) (27.4) 6.2 (4.1) 18.8
Corporate and other expenses (0.3)   (3.0) (1.2)
Net foreign exchange gains (losses) 2.0 (1.7) (2.0) (2.2)
Financing costs (12.5) (15.1) (39.8) (33.1)
Income before income taxes 73.4 160.8 229.4 132.7
Income tax expense (11.7) (30.3) (15.3) (25.0)
Net income 61.7 130.5 214.1 107.7
Losses and loss adjustment expenses incurred - current year (347.2) (196.7) (899.4) (874.2)
Losses and loss adjustment expenses incurred - prior accident years (4.7) 16.0 31.1 (32.4)
Losses and loss adjustment expenses incurred - total $ (351.9) $ (180.7) $ (868.3) $ (906.6)
Underwriting Ratios        
Loss ratio - current year 44.30% 32.80% 46.50% 50.20%
Loss ratio - prior accident years 0.60% (2.70%) (1.60%) 1.90%
Loss ratio - total 44.90% 30.10% 44.90% 52.10%
Policy acquisition expense ratio 30.40% 29.90% 29.80% 29.70%
Underwriting ratio 75.30% 60.00% 74.70% 81.80%
The Fidelis Partnership commissions ratio 12.40% 14.50% 13.10% 13.60%
General and administrative expense ratio 3.80% 4.50% 4.60% 4.10%
Combined ratio 91.50% 79.00% 92.40% 99.50%
Insurance        
Segment Reporting [Line Items]        
Losses and loss adjustment expenses incurred - prior accident years     $ (10.3) $ (103.2)
Reinsurance        
Segment Reporting [Line Items]        
Losses and loss adjustment expenses incurred - prior accident years     41.4 70.8
Operating Segments | Insurance        
Segment Reporting [Line Items]        
Gross premiums written $ 686.1 $ 605.8 3,035.7 2,775.1
Net premiums written 520.5 388.5 2,068.0 1,743.6
Net premiums earned 634.0 456.5 1,663.6 1,427.3
Losses and loss adjustment expenses (341.6) (177.0) (870.6) (750.2)
Policy acquisition expenses (193.8) (137.2) (496.9) (434.3)
General and administrative expenses 0.0 0.0 0.0 0.0
Underwriting income 98.6 142.3 296.1 242.8
Losses and loss adjustment expenses incurred - current year (325.8) (179.7) (860.3) (647.0)
Losses and loss adjustment expenses incurred - prior accident years (15.8) 2.7 (10.3) (103.2)
Losses and loss adjustment expenses incurred - total $ (341.6) $ (177.0) $ (870.6) $ (750.2)
Underwriting Ratios        
Loss ratio - current year 51.40% 39.40% 51.70% 45.40%
Loss ratio - prior accident years 2.50% (0.60%) 0.60% 7.20%
Loss ratio - total 53.90% 38.80% 52.30% 52.60%
Policy acquisition expense ratio 30.60% 30.10% 29.90% 30.40%
Underwriting ratio 84.50% 68.90% 82.20% 83.00%
Operating Segments | Reinsurance        
Segment Reporting [Line Items]        
Gross premiums written $ 153.7 $ 191.7 $ 941.3 $ 964.3
Net premiums written 71.8 115.7 473.8 485.9
Net premiums earned 149.7 143.3 269.7 313.5
Losses and loss adjustment expenses (10.3) (3.7) 2.3 (156.4)
Policy acquisition expenses (44.7) (42.1) (80.0) (81.9)
General and administrative expenses 0.0 0.0 0.0 0.0
Underwriting income 94.7 97.5 192.0 75.2
Losses and loss adjustment expenses incurred - current year (21.4) (17.0) (39.1) (227.2)
Losses and loss adjustment expenses incurred - prior accident years 11.1 13.3 41.4 70.8
Losses and loss adjustment expenses incurred - total $ (10.3) $ (3.7) $ 2.3 $ (156.4)
Underwriting Ratios        
Loss ratio - current year 14.30% 11.90% 14.50% 72.50%
Loss ratio - prior accident years (7.40%) (9.30%) (15.40%) (22.60%)
Loss ratio - total 6.90% 2.60% (0.90%) 49.90%
Policy acquisition expense ratio 29.90% 29.40% 29.70% 26.10%
Underwriting ratio 36.80% 32.00% 28.80% 76.00%
Other        
Segment Reporting [Line Items]        
Gross premiums written $ 0.0 $ 0.0 $ 0.0 $ 0.0
Net premiums written 0.0 0.0 0.0 0.0
Net premiums earned 0.0 0.0 0.0 0.0
Losses and loss adjustment expenses 0.0 0.0 0.0 0.0
Policy acquisition expenses (96.8) (87.1) (254.1) (236.1)
General and administrative expenses (29.5) (27.2) (88.0) (71.5)
Losses and loss adjustment expenses incurred - total $ 0.0 $ 0.0 $ 0.0 $ 0.0