v3.26.3
SEGMENT REPORTING - Schedule of Underwriting Results for Segments (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Dec. 31, 2024
Segment Reporting [Line Items]            
Gross written premiums $ 3,742 $ 4,375 $ 11,116 $ 13,446    
Net written premiums 2,968 3,754 9,192 11,607    
Premiums earned 3,156 3,855 10,220 11,698    
Incurred losses and LAE 2,432 2,837 6,811 8,203    
Commission and brokerage 747 890 2,386 2,595    
Other underwriting expenses 200 258 641 750    
Underwriting income (loss) (223) (130) 383 151    
Ceded prior year (unfavorable) favorable development and related amortization under ADC 0 0 (8) 0    
Net investment income 634 540 1,725 1,563    
Net gains (losses) on investments (68) (47) (86) (59)    
Corporate expenses (30) (27) (101) (79)    
Interest, fee and bond issue cost amortization expense (36) (38) (107) (114)    
Other income (expense) (9) (29) (117) (129)    
Income tax expense (benefit) (36) (14) (243) (187)    
NET INCOME (LOSS) 233 255 1,445 1,145    
Reserve for losses and loss adjustment expenses 35,288 33,742 35,288 33,742 $ 34,312 $ 29,889
Reinsurance Treaty | Operating Segments            
Segment Reporting [Line Items]            
Gross written premiums 2,715 2,867 8,109 8,753    
Net written premiums 2,164 2,548 6,797 7,861    
Premiums earned 2,243 2,579 7,158 7,885    
Incurred losses and LAE 1,797 1,490 4,723 5,063    
Commission and brokerage 572 692 1,830 2,010    
Other underwriting expenses 65 62 197 186    
Underwriting income (loss) (191) 335 407 626    
Global Wholesale & Specialty | Operating Segments            
Segment Reporting [Line Items]            
Gross written premiums 948 918 2,699 2,646    
Net written premiums 747 761 2,177 2,182    
Premiums earned 705 737 2,133 2,197    
Incurred losses and LAE 449 465 1,332 1,417    
Commission and brokerage 149 149 448 440    
Other underwriting expenses 89 90 268 240    
Underwriting income (loss) 19 33 85 100    
Legacy            
Segment Reporting [Line Items]            
Reserve for losses and loss adjustment expenses 7,790 8,379 7,790 8,379    
Less: unpaid reinsurance loss recoverables before impact of ADC (1,907) (2,073) (1,907) (2,073)    
Net loss and loss adjustment expense reserves before impact of ADC 5,882 6,306 5,882 6,306    
Legacy | Operating Segments            
Segment Reporting [Line Items]            
Gross written premiums 79 590 308 2,048    
Net written premiums 57 444 218 1,565    
Premiums earned 208 539 929 1,616    
Incurred losses and LAE 186 882 755 1,723    
Commission and brokerage 26 50 108 144    
Other underwriting expenses 46 105 175 324    
Underwriting income (loss) $ (51) $ (498) $ (109) $ (575)