Shareholders' Equity - Schedule of Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 8,008	$ 6,028	$ 6,305	$ (742)
Other comprehensive income (loss) before reclassifications, net			754	174
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)			102	105
Tax expense			(26)	(27)
Amounts reclassified from accumulated other comprehensive income (loss), net			76	78
Total other comprehensive income (expense) attributable to Aon shareholders	(72)	357	830	252
Ending balance	8,114	6,418	8,114	6,418
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(3,843)	(4,478)	(4,745)	(4,373)
Amounts reclassified from accumulated other comprehensive income
Ending balance	(3,915)	(4,121)	(3,915)	(4,121)
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			10	85
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)			(8)	(1)
Tax expense			2	0
Amounts reclassified from accumulated other comprehensive income (loss), net			(6)	(1)
Total other comprehensive income (expense) attributable to Aon shareholders			4	84
Ending balance	78	86	78	86
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			752	129
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 (expense) attributable to Aon shareholders			752	129
Ending balance	(1,299)	(1,455)	(1,299)	(1,455)
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			(8)	(40)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)			110	106
Tax expense			(28)	(27)
Amounts reclassified from accumulated other comprehensive income (loss), net			82	79
Total other comprehensive income (expense) attributable to Aon shareholders			74	39
Ending balance	$ (2,694)	$ (2,752)	$ (2,694)	$ (2,752)