v3.26.3
Shareholders' Equity - Schedule of Components of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]        
Beginning balance $ 9,686 $ 8,008 $ 9,459 $ 6,305
Other comprehensive income (loss) before reclassifications, net     (145) 754
Amounts reclassified from accumulated other comprehensive income        
Amounts reclassified from accumulated other comprehensive income (loss)     110 102
Tax expense     (27) (26)
Amounts reclassified from accumulated other comprehensive income (loss), net     83 76
Total other comprehensive income (loss) attributable to Aon shareholders 81 (72) (62) 830
Ending balance 10,197 8,114 10,197 8,114
AOCI Attributable to Parent        
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]        
Beginning balance (3,986) (3,843) (3,843) (4,745)
Amounts reclassified from accumulated other comprehensive income        
Ending balance (3,905) (3,915) (3,905) (3,915)
Change in Fair Value of Financial Instruments        
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]        
Beginning balance     78 74
Other comprehensive income (loss) before reclassifications, net     19 10
Amounts reclassified from accumulated other comprehensive income        
Amounts reclassified from accumulated other comprehensive income (loss)     (3) (8)
Tax expense     1 2
Amounts reclassified from accumulated other comprehensive income (loss), net     (2) (6)
Total other comprehensive income (loss) attributable to Aon shareholders     17 4
Ending balance 95 78 95 78
Foreign Currency Translation Adjustments        
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]        
Beginning balance     (1,225) (2,051)
Other comprehensive income (loss) before reclassifications, net     (159) 752
Amounts reclassified from accumulated other comprehensive income        
Amounts reclassified from accumulated other comprehensive income (loss)     2 0
Tax expense     0 0
Amounts reclassified from accumulated other comprehensive income (loss), net     2 0
Total other comprehensive income (loss) attributable to Aon shareholders     (157) 752
Ending balance (1,382) (1,299) (1,382) (1,299)
Postretirement Benefit Obligation        
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]        
Beginning balance     (2,696) (2,768)
Other comprehensive income (loss) before reclassifications, net     (5) (8)
Amounts reclassified from accumulated other comprehensive income        
Amounts reclassified from accumulated other comprehensive income (loss)     111 110
Tax expense     (28) (28)
Amounts reclassified from accumulated other comprehensive income (loss), net     83 82
Total other comprehensive income (loss) attributable to Aon shareholders     78 74
Ending balance $ (2,618) $ (2,694) $ (2,618) $ (2,694)