v3.26.1
Segments - Schedule of Group Segment Disclosures (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting [Line Items]        
Gross premiums written $ 1,296.8 $ 1,219.0 $ 3,137.2 $ 2,941.9
Net premiums written 769.2 698.9 1,949.5 1,725.3
Net premiums earned 581.1 538.0 1,149.6 1,141.0
Losses and loss adjustment expenses (292.6) (296.7) (516.4) (725.9)
Policy acquisition expenses (256.4) (239.6) (495.7) (485.9)
General and administrative expenses (29.3) (22.3) (58.5) (44.3)
Underwriting income/(loss) 2.8 (20.6) 79.0 (115.1)
Net investment income 44.0 44.6 87.7 94.1
Net realized and unrealized investment gains 24.9 6.7 23.3 12.6
Corporate and other expenses (2.3) (1.2) (2.7) (1.2)
Net foreign exchange gains (losses) (3.9) 2.0 (4.0) (0.5)
Financing costs (12.6) (9.3) (27.3) (18.0)
Income/(loss) before income taxes 52.9 22.2 156.0 (28.1)
Income tax expense (benefit) (8.5) (2.5) (3.6) 5.3
Net income/(loss) 44.4 19.7 152.4 (22.8)
Losses and loss adjustment expenses incurred - current year (325.3) (207.5) (552.2) (677.5)
Losses and loss adjustment expenses incurred - prior accident years 32.7 (89.2) 35.8 (48.4)
Losses and loss adjustment expenses incurred - total $ (292.6) $ (296.7) $ (516.4) $ (725.9)
Underwriting Ratios        
Loss ratio - current year 56.00% 38.50% 48.00% 59.40%
Loss ratio - prior accident years (5.60%) 16.60% (3.10%) 4.20%
Loss ratio - total 50.40% 55.10% 44.90% 63.60%
Policy acquisition expense ratio 32.00% 31.40% 29.40% 29.50%
Underwriting ratio 82.40% 86.50% 74.30% 93.10%
The Fidelis Partnership commissions ratio 12.10% 13.10% 13.70% 13.10%
General and administrative expense ratio 5.00% 4.10% 5.10% 3.90%
Combined ratio 99.50% 103.70% 93.10% 110.10%
Insurance        
Segment Reporting [Line Items]        
Losses and loss adjustment expenses incurred - prior accident years     $ 5.5 $ (105.9)
Reinsurance        
Segment Reporting [Line Items]        
Losses and loss adjustment expenses incurred - prior accident years     30.3 57.5
Operating Segments | Insurance        
Segment Reporting [Line Items]        
Gross premiums written $ 913.5 $ 902.3 2,349.6 2,169.3
Net premiums written 543.5 546.2 1,547.5 1,355.1
Net premiums earned 514.7 458.9 1,029.6 970.8
Losses and loss adjustment expenses (301.2) (291.8) (529.0) (573.2)
Policy acquisition expenses (163.9) (148.9) (303.1) (297.1)
General and administrative expenses 0.0 0.0 0.0 0.0
Underwriting income/(loss) 49.6 18.2 197.5 100.5
Losses and loss adjustment expenses incurred - current year (321.9) (178.4) (534.5) (467.3)
Losses and loss adjustment expenses incurred - prior accident years 20.7 (113.4) 5.5 (105.9)
Losses and loss adjustment expenses incurred - total $ (301.2) $ (291.8) $ (529.0) $ (573.2)
Underwriting Ratios        
Loss ratio - current year 62.50% 38.90% 51.90% 48.10%
Loss ratio - prior accident years (4.00%) 24.70% (0.50%) 10.90%
Loss ratio - total 58.50% 63.60% 51.40% 59.00%
Policy acquisition expense ratio 31.80% 32.40% 29.40% 30.60%
Underwriting ratio 90.30% 96.00% 80.80% 89.60%
Operating Segments | Reinsurance        
Segment Reporting [Line Items]        
Gross premiums written $ 383.3 $ 316.7 $ 787.6 $ 772.6
Net premiums written 225.7 152.7 402.0 370.2
Net premiums earned 66.4 79.1 120.0 170.2
Losses and loss adjustment expenses 8.6 (4.9) 12.6 (152.7)
Policy acquisition expenses (22.0) (20.1) (35.3) (39.8)
General and administrative expenses 0.0 0.0 0.0 0.0
Underwriting income/(loss) 53.0 54.1 97.3 (22.3)
Losses and loss adjustment expenses incurred - current year (3.4) (29.1) (17.7) (210.2)
Losses and loss adjustment expenses incurred - prior accident years 12.0 24.2 30.3 57.5
Losses and loss adjustment expenses incurred - total $ 8.6 $ (4.9) $ 12.6 $ (152.7)
Underwriting Ratios        
Loss ratio - current year 5.10% 36.80% 14.80% 123.50%
Loss ratio - prior accident years (18.10%) (30.60%) (25.30%) (33.80%)
Loss ratio - total (13.00%) 6.20% (10.50%) 89.70%
Policy acquisition expense ratio 33.10% 25.40% 29.40% 23.40%
Underwriting ratio 20.10% 31.60% 18.90% 113.10%
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 (70.5) (70.6) (157.3) (149.0)
General and administrative expenses (29.3) (22.3) (58.5) (44.3)
Losses and loss adjustment expenses incurred - total $ 0.0 $ 0.0 $ 0.0 $ 0.0