v3.25.1
Shareholders' Equity (Deficit) - Schedule of Components of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]    
Beginning balance $ 6,305 $ (742)
Other comprehensive income (loss) before reclassifications, net 261 (60)
Amounts reclassified from accumulated other comprehensive income    
Amounts reclassified from accumulated other comprehensive income (loss) 38 39
Tax expense (10) (10)
Amounts reclassified from accumulated other comprehensive income (loss), net 28 29
Total other comprehensive income (loss) attributable to Aon shareholders 289 (31)
Ending balance 7,195 (28)
AOCI Attributable to Parent    
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]    
Beginning balance (4,745) (4,373)
Amounts reclassified from accumulated other comprehensive income    
Ending balance (4,456) (4,404)
Change in Fair Value of Financial Instruments    
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]    
Beginning balance 74 2
Other comprehensive income (loss) before reclassifications, net 2 72
Amounts reclassified from accumulated other comprehensive income    
Amounts reclassified from accumulated other comprehensive income (loss) 2 4
Tax expense (1) (1)
Amounts reclassified from accumulated other comprehensive income (loss), net 1 3
Total other comprehensive income (loss) attributable to Aon shareholders 3 75
Ending balance 77 77
Foreign Currency Translation Adjustments    
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]    
Beginning balance (2,051) (1,584)
Other comprehensive income (loss) before reclassifications, net 239 (132)
Amounts reclassified from accumulated other comprehensive income    
Amounts reclassified from accumulated other comprehensive income (loss) 0 0
Tax expense 0 0
Amounts reclassified from accumulated other comprehensive income (loss), net 0 0
Total other comprehensive income (loss) attributable to Aon shareholders 239 (132)
Ending balance (1,812) (1,716)
Postretirement Benefit Obligation    
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]    
Beginning balance (2,768) (2,791)
Other comprehensive income (loss) before reclassifications, net 20 0
Amounts reclassified from accumulated other comprehensive income    
Amounts reclassified from accumulated other comprehensive income (loss) 36 35
Tax expense (9) (9)
Amounts reclassified from accumulated other comprehensive income (loss), net 27 26
Total other comprehensive income (loss) attributable to Aon shareholders 47 26
Ending balance $ (2,721) $ (2,765)