Shareholders’ Equity - Schedule of Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 9,459	$ 6,305
Amounts reclassified from accumulated other comprehensive income
Total other comprehensive income (loss) attributable to Aon shareholders	(85)	289
Ending balance	9,958	7,195
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(3,843)	(4,745)
Other comprehensive income (loss) before reclassifications, net	(111)	261
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)	35	38
Tax expense	(9)	(10)
Amounts reclassified from accumulated other comprehensive income (loss), net	26	28
Total other comprehensive income (loss) attributable to Aon shareholders	(85)	289
Ending balance	(3,928)	(4,456)
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	(3)	2
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)	(1)	2
Tax expense	0	(1)
Amounts reclassified from accumulated other comprehensive income (loss), net	(1)	1
Total other comprehensive income (loss) attributable to Aon shareholders	(4)	3
Ending balance	74	77
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	(108)	239
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	(108)	239
Ending balance	(1,333)	(1,812)
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	0	20
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)	36	36
Tax expense	(9)	(9)
Amounts reclassified from accumulated other comprehensive income (loss), net	27	27
Total other comprehensive income (loss) attributable to Aon shareholders	27	47
Ending balance	$ (2,669)	$ (2,721)