Shareholders' Equity - Schedule of Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 7,195	$ (28)	$ 6,305	$ (742)
Other comprehensive income (loss) before reclassifications, net			853	(158)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)			66	72
Tax expense			(17)	(19)
Amounts reclassified from accumulated other comprehensive income (loss), net			49	53
Total other comprehensive income (loss) attributable to Aon shareholders	613	(74)	902	(105)
Ending balance	8,008	6,028	8,008	6,028
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(4,456)	(4,404)	(4,745)	(4,373)
Amounts reclassified from accumulated other comprehensive income
Ending balance	(3,843)	(4,478)	(3,843)	(4,478)
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			17	75
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)			(6)	2
Tax expense			2	(1)
Amounts reclassified from accumulated other comprehensive income (loss), net			(4)	1
Total other comprehensive income (loss) attributable to Aon shareholders			13	76
Ending balance	87	78	87	78
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			843	(220)
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			843	(220)
Ending balance	(1,208)	(1,804)	(1,208)	(1,804)
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			(7)	(13)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)			72	70
Tax expense			(19)	(18)
Amounts reclassified from accumulated other comprehensive income (loss), net			53	52
Total other comprehensive income (loss) attributable to Aon shareholders			46	39
Ending balance	$ (2,722)	$ (2,752)	$ (2,722)	$ (2,752)