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Segment Information - Summary of underwriting income or loss by segment (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2025
USD ($)
segment
Mar. 31, 2024
USD ($)
Dec. 31, 2024
USD ($)
Segment Reporting Information [Line Items]      
Number of segments | segment 3    
Gross premiums written $ 6,463 $ 5,933  
Premiums ceded (1,948) (1,848)  
Net premiums written 4,515 4,085  
Change in unearned premiums (327) (663)  
Net premiums earned 4,188 3,422  
Other underwriting income 53 12  
Losses and loss adjustment expenses (2,587) (1,728)  
Acquisition expenses (764) (607)  
Other operating expenses (473) (363)  
Underwriting income (loss) 417 736  
Net investment income 378 327  
Net realized gains (losses) 3 67  
Equity in net income of investments accounted for using the equity method 53 99  
Other income (loss) (2) 14  
Corporate expenses [1] (50) (46)  
Transaction costs and other [1] (10) (7)  
Amortization of intangible assets (49) (21)  
Interest expense (35) (34)  
Net foreign exchange gains (losses) (27) 31  
Income (loss) before income taxes and income (loss) from operating affiliates 678 1,166  
Income tax expense (benefit) (121) (101)  
Income (loss) from operating affiliates 17 55  
Net income available to Arch 574 1,120  
Preferred dividends (10) (10)  
Net income (loss) available to Arch common shareholders $ 564 $ 1,110  
Underwriting Ratios      
Loss ratio 61.80% 50.50%  
Acquisition expense ratio 18.30% 17.70%  
Other operating expense ratio [2] 10.00%    
Other operating expense ratio   10.60%  
Combined ratio 90.10% 78.80%  
Goodwill and intangible assets $ 1,308 $ 778 $ 1,351
Operating segments | Insurance      
Segment Reporting Information [Line Items]      
Gross premiums written [3] 2,645 2,126  
Premiums ceded [3] (712) (584)  
Net premiums written 1,933 1,542  
Change in unearned premiums (73) (91)  
Net premiums earned 1,860 1,451  
Other underwriting income 3 [4] 0  
Losses and loss adjustment expenses (1,228) (854)  
Acquisition expenses (343) (276)  
Other operating expenses [5] (294) [4] (235)  
Underwriting income (loss) $ (2) $ 86  
Underwriting Ratios      
Loss ratio 66.00% 58.90%  
Acquisition expense ratio 18.50% 19.00%  
Other operating expense ratio [2] 15.60%    
Other operating expense ratio   16.20%  
Combined ratio 100.10% 94.10%  
Goodwill and intangible assets $ 878 $ 293  
Operating segments | Reinsurance      
Segment Reporting Information [Line Items]      
Gross premiums written [3] 3,494 3,467  
Premiums ceded [3] (1,178) (1,201)  
Net premiums written 2,316 2,266  
Change in unearned premiums (288) (600)  
Net premiums earned 2,028 1,666  
Other underwriting income 39 [4] 2  
Losses and loss adjustment expenses (1,356) (883)  
Acquisition expenses (417) (331)  
Other operating expenses [5] (127) [4] (75)  
Underwriting income (loss) $ 167 $ 379  
Underwriting Ratios      
Loss ratio 66.90% 53.00%  
Acquisition expense ratio 20.60% 19.90%  
Other operating expense ratio [2] 4.30%    
Other operating expense ratio   4.50%  
Combined ratio 91.80% 77.40%  
Goodwill and intangible assets $ 102 $ 121  
Operating segments | Mortgage      
Segment Reporting Information [Line Items]      
Gross premiums written [3] 326 341  
Premiums ceded [3] (60) (64)  
Net premiums written 266 277  
Change in unearned premiums 34 28  
Net premiums earned 300 305  
Other underwriting income 11 [4] 10  
Losses and loss adjustment expenses (3) 9  
Acquisition expenses (4) 0  
Other operating expenses [5] (52) [4] (53)  
Underwriting income (loss) $ 252 $ 271  
Underwriting Ratios      
Loss ratio 1.10% (3.00%)  
Acquisition expense ratio 1.30% 0.00%  
Other operating expense ratio [2] 13.70%    
Other operating expense ratio   17.50%  
Combined ratio 16.10% 14.50%  
Goodwill and intangible assets $ 328 $ 364  
[1] Certain expenses have been excluded from ‘Corporate expenses’ and reflected in ‘Transaction costs and other.’
[2] The ‘Other operating expense ratio’ for the 2025 period includes ‘Other underwriting income (loss).’
[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 (loss)’ 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.