v3.26.1
Segment Information - Summary of underwriting income or loss by segment (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
USD ($)
Jun. 30, 2025
USD ($)
Jun. 30, 2026
USD ($)
segment
Jun. 30, 2025
USD ($)
Dec. 31, 2025
USD ($)
Segment Reporting [Line Items]          
Number of reportable segments | segment     3    
Gross premiums written $ 6,126 $ 6,196 $ 12,551 $ 12,659  
Premiums ceded (2,077) (1,848) (4,154) (3,796)  
Net premiums written 4,049 4,348 8,397 8,863  
Change in unearned premiums (64) (11) (426) (338)  
Net premiums earned 3,985 4,337 7,971 8,525  
Other underwriting income 57 62 116 115  
Losses and loss adjustment expenses (2,196) (2,303) (4,285) (4,890)  
Acquisition expenses (718) (824) (1,448) (1,588)  
Other operating expenses (471) (454) (969) (927)  
Underwriting income (loss) 657 818 1,385 1,235  
Net investment income 417 405 825 783  
Net realized gains (losses) (17) 229 (104) 232  
Equity in net income of investments accounted for using the equity method 196 162 356 215  
Other income (loss) 30 18 25 16  
Corporate expenses [1] (12) (29) (43) (79)  
Transaction costs and other [1] (32) (18) (50) (28)  
Amortization of intangible assets (30) (48) (60) (97)  
Interest expense (44) (38) (81) (73)  
Net foreign exchange gains (losses) 10 (88) 31 (115)  
Income (loss) before income taxes and income (loss) from operating affiliates 1,175 1,411 2,284 2,089  
Income tax expense (benefit) (164) (214) (262) (335)  
Income (loss) from operating affiliates 46 40 82 57  
Net income available to Arch 1,057 1,237 2,104 1,811  
Preferred dividends (10) (10) (20) (20)  
Net income (loss) available to Arch common shareholders $ 1,047 $ 1,227 $ 2,084 $ 1,791  
Underwriting Ratios          
Loss ratio 55.10% 53.10% 53.80% 57.40%  
Acquisition expense ratio 18.00% 19.00% 18.20% 18.60%  
Other operating expense ratio [2] 10.40% 9.10% 10.70% 9.50%  
Combined ratio 83.50% 81.20% 82.70% 85.50%  
Goodwill and intangible assets $ 1,163 $ 1,319 $ 1,163 $ 1,319 $ 1,222
Operating segments | Insurance          
Segment Reporting [Line Items]          
Gross premiums written [3] 2,603 2,681 5,300 5,326  
Premiums ceded [3] (670) (645) (1,461) (1,357)  
Net premiums written 1,933 2,036 3,839 3,969  
Change in unearned premiums (53) (67) (88) (140)  
Net premiums earned 1,880 1,969 3,751 3,829  
Other underwriting income [4] 15 13 26 16  
Losses and loss adjustment expenses (1,185) (1,178) (2,311) (2,406)  
Acquisition expenses (375) (387) (750) (730)  
Other operating expenses [5] (308) (288) (623) (582)  
Underwriting income (loss) $ 27 $ 129 $ 93 $ 127  
Underwriting Ratios          
Loss ratio 63.00% 59.80% 61.60% 62.80%  
Acquisition expense ratio 19.90% 19.60% 20.00% 19.10%  
Other operating expense ratio [2] 15.60% 14.00% 15.90% 14.80%  
Combined ratio 98.50% 93.40% 97.50% 96.70%  
Goodwill and intangible assets $ 747 $ 875 $ 747 $ 875  
Operating segments | Reinsurance          
Segment Reporting [Line Items]          
Gross premiums written [3] 3,202 3,196 6,616 6,690  
Premiums ceded [3] (1,358) (1,137) (2,596) (2,315)  
Net premiums written 1,844 2,059 4,020 4,375  
Change in unearned premiums (24) 28 (369) (260)  
Net premiums earned 1,820 2,087 3,651 4,115  
Other underwriting income [4] 37 46 74 85  
Losses and loss adjustment expenses (992) (1,128) (1,940) (2,484)  
Acquisition expenses (341) (436) (688) (853)  
Other operating expenses [5] (114) (118) (246) (245)  
Underwriting income (loss) $ 410 $ 451 $ 851 $ 618  
Underwriting Ratios          
Loss ratio 54.60% 54.10% 53.10% 60.40%  
Acquisition expense ratio 18.70% 20.90% 18.80% 20.70%  
Other operating expense ratio [2] 4.20% 3.50% 4.70% 3.90%  
Combined ratio 77.50% 78.50% 76.60% 85.00%  
Goodwill and intangible assets $ 91 $ 105 $ 91 $ 105  
Operating segments | Mortgage          
Segment Reporting [Line Items]          
Gross premiums written [3] 324 323 640 649  
Premiums ceded [3] (52) (70) (102) (130)  
Net premiums written 272 253 538 519  
Change in unearned premiums 13 28 31 62  
Net premiums earned 285 281 569 581  
Other underwriting income [4] 5 3 16 14  
Losses and loss adjustment expenses (19) 3 (34) 0  
Acquisition expenses (2) (1) (10) (5)  
Other operating expenses [5] (49) (48) (100) (100)  
Underwriting income (loss) $ 220 $ 238 $ 441 $ 490  
Underwriting Ratios          
Loss ratio 6.50% (1.20%) 5.90% 0.00%  
Acquisition expense ratio 0.90% 0.40% 1.90% 0.90%  
Other operating expense ratio [2] 15.40% 16.00% 14.80% 14.90%  
Combined ratio 22.80% 15.20% 22.60% 15.80%  
Goodwill and intangible assets $ 325 $ 339 $ 325 $ 339  
[1] Certain expenses have been excluded from ‘Corporate expenses’ and reflected in ‘Transaction costs and other.’
[2] The ‘Other operating expense ratio’ includes ‘Other underwriting income.’
[3] Certain assumed and ceded amounts related to intersegment transactions are included in individual segment results. Accordingly, the sum of such transactions for each segment does not agree to the total due to eliminations.
[4] ‘Other underwriting income’ includes revenue earned from underwriting-related activities covered under existing service contracts.
[5] ‘Other operating expenses’ primarily include expenses that are related to compensation and employee benefits, information technology and professional fees.