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Segment Information - Summary of net premiums written and earned by major line of business and net premiums written by location (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Segment Reporting Information [Line Items]      
Net premiums earned $ 12,440 $ 9,679 $ 8,082
Net premiums written 13,468 11,078 9,017
Operating segments | Insurance      
Segment Reporting Information [Line Items]      
Net premiums earned 5,446 4,560 3,625
Net premiums written 5,862 5,021 4,149
Operating segments | Insurance | United States      
Segment Reporting Information [Line Items]      
Net premiums written [1] 3,780 3,342 2,812
Operating segments | Insurance | Europe      
Segment Reporting Information [Line Items]      
Net premiums written [1] 1,774 1,405 1,126
Operating segments | Insurance | Other geographic location      
Segment Reporting Information [Line Items]      
Net premiums written [1] 308 274 211
Operating segments | Insurance | Professional lines      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 1,419 1,314 943
Operating segments | Insurance | Property, energy, marine and aviation      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 1,064 772 668
Operating segments | Insurance | Programs      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 658 590 507
Operating segments | Insurance | Construction and national accounts      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 561 432 417
Operating segments | Insurance | Travel, accident and health      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 557 492 256
Operating segments | Insurance | Excess and surplus casualty      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 486 393 318
Operating segments | Insurance | Warranty and lenders solutions      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 185 128 153
Operating segments | Insurance | Other      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 516 439 363
Operating segments | Reinsurance      
Segment Reporting Information [Line Items]      
Net premiums earned 5,836 3,959 2,841
Net premiums written 6,554 4,924 3,254
Operating segments | Reinsurance | United States      
Segment Reporting Information [Line Items]      
Net premiums written [1] 1,756 1,247 829
Operating segments | Reinsurance | Bermuda      
Segment Reporting Information [Line Items]      
Net premiums written [1] 3,288 2,561 1,555
Operating segments | Reinsurance | Europe and other      
Segment Reporting Information [Line Items]      
Net premiums written [1] 1,510 1,116 870
Operating segments | Reinsurance | Other specialty      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 2,097 1,378 819
Operating segments | Reinsurance | Property excluding property catastrophe      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 1,645 1,090 838
Operating segments | Reinsurance | Casualty      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 1,005 855 667
Operating segments | Reinsurance | Property catastrophe      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 742 367 280
Operating segments | Reinsurance | Marine and aviation      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 229 159 153
Operating segments | Reinsurance | Other      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 118 110 84
Operating segments | Mortgage      
Segment Reporting Information [Line Items]      
Net premiums earned 1,158 1,160 1,283
Net premiums written 1,052 1,133 1,261
Operating segments | Mortgage | United States      
Segment Reporting Information [Line Items]      
Net premiums written 743 781 913
Operating segments | Mortgage | Other geographic location      
Segment Reporting Information [Line Items]      
Net premiums written 309 352 348
Operating segments | Mortgage | US Primary Mortgage Insurance      
Segment Reporting Information [Line Items]      
Net premiums earned 759 804 953
Operating segments | Mortgage | US Credit Risk Transfer And Other      
Segment Reporting Information [Line Items]      
Net premiums earned 220 196 181
Operating segments | Mortgage | International Mortgage Insurance And Reinsurance      
Segment Reporting Information [Line Items]      
Net premiums earned 179 160 149
Operating segments | Other      
Segment Reporting Information [Line Items]      
Net premiums earned 0 0 333
Net premiums written 0 0 353
Operating segments | Other | United States      
Segment Reporting Information [Line Items]      
Net premiums written [1] 0 0 63
Operating segments | Other | Europe      
Segment Reporting Information [Line Items]      
Net premiums written [1] 0 0 91
Operating segments | Other | Bermuda      
Segment Reporting Information [Line Items]      
Net premiums written [1] 0 0 199
Operating segments | Other | Other specialty      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 0 0 118
Operating segments | Other | Property excluding property catastrophe      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 0 0 7
Operating segments | Other | Casualty      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 0 0 138
Operating segments | Other | Property catastrophe      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 0 0 15
Operating segments | Other | Marine and aviation      
Segment Reporting Information [Line Items]      
Net premiums earned [1] 0 0 0
Operating segments | Other | Other      
Segment Reporting Information [Line Items]      
Net premiums earned [1] $ 0 $ 0 $ 55
[1] Segment results include intersegment transactions.