Shareholders' Equity (Deficit) - Schedule of Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 6,028	$ 162	$ (742)	$ (429)
Other comprehensive income (loss) before reclassifications, net			174	(53)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)			105	131
Tax expense			(27)	(34)
Amounts reclassified from accumulated other comprehensive income (loss), net			78	97
Total other comprehensive income (loss) attributable to Aon shareholders	357	(241)	252	44
Ending balance	6,418	(486)	6,418	(486)
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(4,478)	(4,338)	(4,373)	(4,623)
Amounts reclassified from accumulated other comprehensive income
Ending balance	(4,121)	(4,579)	(4,121)	(4,579)
Change in Fair Value of Financial Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			2	(11)
Other comprehensive income (loss) before reclassifications, net			85	1
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)			(1)	7
Tax expense			0	(2)
Amounts reclassified from accumulated other comprehensive income (loss), net			(1)	5
Total other comprehensive income (loss) attributable to Aon shareholders			84	6
Ending balance	86	(5)	86	(5)
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(1,584)	(1,861)
Other comprehensive income (loss) before reclassifications, net			129	(28)
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			129	(28)
Ending balance	(1,455)	(1,889)	(1,455)	(1,889)
Postretirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(2,791)	(2,751)
Other comprehensive income (loss) before reclassifications, net			(40)	(26)
Amounts reclassified from accumulated other comprehensive income
Amounts reclassified from accumulated other comprehensive income (loss)			106	124
Tax expense			(27)	(32)
Amounts reclassified from accumulated other comprehensive income (loss), net			79	92
Total other comprehensive income (loss) attributable to Aon shareholders			39	66
Ending balance	$ (2,752)	$ (2,685)	$ (2,752)	$ (2,685)