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Segment Information - Summary of underwriting income or loss by segment (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2025
USD ($)
Jun. 30, 2024
USD ($)
Jun. 30, 2025
USD ($)
segment
Jun. 30, 2024
USD ($)
Dec. 31, 2024
USD ($)
Segment Reporting Information [Line Items]          
Number of segments | segment     3    
Gross premiums written $ 6,196 $ 5,382 $ 12,659 $ 11,315  
Premiums ceded (1,848) (1,601) (3,796) (3,449)  
Net premiums written 4,348 3,781 8,863 7,866  
Change in unearned premiums (11) (216) (338) (879)  
Net premiums earned 4,337 3,565 8,525 6,987  
Other underwriting income 62 3 115 15  
Losses and loss adjustment expenses (2,303) (1,827) (4,890) (3,555)  
Acquisition expenses (824) (633) (1,588) (1,240)  
Other operating expenses (454) (346) (927) (709)  
Underwriting income (loss) 818 762 1,235 1,498  
Net investment income 405 364 783 691  
Net realized gains (losses) 229 122 232 189  
Equity in net income of investments accounted for using the equity method 162 167 215 266  
Other income (loss) 18 8 16 22  
Corporate expenses [1] (29) (23) (79) (69)  
Transaction costs and other [1] (18) (18) (28) (25)  
Amortization of intangible assets (48) (27) (97) (48)  
Interest expense (38) (35) (73) (69)  
Net foreign exchange gains (losses) (88) 1 (115) 32  
Income (loss) before income taxes and income (loss) from operating affiliates 1,411 1,321 2,089 2,487  
Income tax expense (benefit) (214) (97) (335) (198)  
Income (loss) from operating affiliates 40 45 57 100  
Net income available to Arch 1,237 1,269 1,811 2,389  
Preferred dividends (10) (10) (20) (20)  
Net income (loss) available to Arch common shareholders $ 1,227 $ 1,259 $ 1,791 $ 2,369  
Underwriting Ratios          
Loss ratio 53.10% 51.20% 57.40% 50.90%  
Acquisition expense ratio 19.00% 17.80% 18.60% 17.70%  
Other operating expense ratio [2] 9.10%   9.50%    
Other operating expense ratio   9.70%   10.10%  
Combined ratio 81.20% 78.70% 85.50% 78.70%  
Goodwill and intangible assets $ 1,319 $ 725 $ 1,319 $ 725 $ 1,351
Operating segments | Insurance          
Segment Reporting Information [Line Items]          
Gross premiums written [3] 2,681 2,102 5,326 4,228  
Premiums ceded [3] (645) (544) (1,357) (1,128)  
Net premiums written 2,036 1,558 3,969 3,100  
Change in unearned premiums (67) (80) (140) (171)  
Net premiums earned 1,969 1,478 3,829 2,929  
Other underwriting income 13 [4] 0 16 [4] 0  
Losses and loss adjustment expenses (1,178) (848) (2,406) (1,702)  
Acquisition expenses (387) (288) (730) (564)  
Other operating expenses [5] (288) (233) (582) (468)  
Underwriting income (loss) $ 129 $ 109 $ 127 $ 195  
Underwriting Ratios          
Loss ratio 59.80% 57.30% 62.80% 58.10%  
Acquisition expense ratio 19.60% 19.50% 19.10% 19.20%  
Other operating expense ratio [2] 14.00%   14.80%    
Other operating expense ratio   15.80%   16.00%  
Combined ratio 93.40% 92.60% 96.70% 93.30%  
Goodwill and intangible assets $ 875 $ 255 $ 875 $ 255  
Operating segments | Reinsurance          
Segment Reporting Information [Line Items]          
Gross premiums written [3] 3,196 2,941 6,690 6,408  
Premiums ceded [3] (1,137) (994) (2,315) (2,195)  
Net premiums written 2,059 1,947 4,375 4,213  
Change in unearned premiums 28 (167) (260) (767)  
Net premiums earned 2,087 1,780 4,115 3,446  
Other underwriting income 46 [4] 1 85 [4] 3  
Losses and loss adjustment expenses (1,128) (1,006) (2,484) (1,889)  
Acquisition expenses (436) (345) (853) (676)  
Other operating expenses [5] (118) (64) (245) (139)  
Underwriting income (loss) $ 451 $ 366 $ 618 $ 745  
Underwriting Ratios          
Loss ratio 54.10% 56.50% 60.40% 54.80%  
Acquisition expense ratio 20.90% 19.40% 20.70% 19.60%  
Other operating expense ratio [2] 3.50%   3.90%    
Other operating expense ratio   3.60%   4.00%  
Combined ratio 78.50% 79.50% 85.00% 78.40%  
Goodwill and intangible assets $ 105 $ 114 $ 105 $ 114  
Operating segments | Mortgage          
Segment Reporting Information [Line Items]          
Gross premiums written [3] 323 340 649 681  
Premiums ceded [3] (70) (64) (130) (128)  
Net premiums written 253 276 519 553  
Change in unearned premiums 28 31 62 59  
Net premiums earned 281 307 581 612  
Other underwriting income 3 [4] 2 14 [4] 12  
Losses and loss adjustment expenses 3 27 0 36  
Acquisition expenses (1) 0 (5) 0  
Other operating expenses [5] (48) (49) (100) (102)  
Underwriting income (loss) $ 238 $ 287 $ 490 $ 558  
Underwriting Ratios          
Loss ratio (1.20%) (8.60%) 0.00% (5.80%)  
Acquisition expense ratio 0.40% 0.10% 0.90% 0.10%  
Other operating expense ratio [2] 16.00%   14.90%    
Other operating expense ratio   15.90%   16.70%  
Combined ratio 15.20% 7.40% 15.80% 11.00%  
Goodwill and intangible assets $ 339 $ 356 $ 339 $ 356  
[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.’
[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.