Shareholders' Equity (Deficit) - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 162	$ 788	$ (429)	$ 1,158
Other comprehensive income (loss) before reclassifications, net			(53)	(1,112)
Amounts reclassified from accumulated other comprehensive income
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent			131	87
Tax expense			(34)	(23)
Amounts reclassified from accumulated other comprehensive income, net			97	64
Total other comprehensive income (loss) attributable to Aon shareholders	(241)	(659)	44	(1,048)
Ending balance	(486)	(670)	(486)	(670)
AOCI Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(4,338)	(4,260)	(4,623)	(3,871)
Amounts reclassified from accumulated other comprehensive income
Ending balance	(4,579)	(4,919)	(4,579)	(4,919)
Change in Fair Value of Financial Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(11)	2
Other comprehensive income (loss) before reclassifications, net			1	(13)
Amounts reclassified from accumulated other comprehensive income
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent			7	(3)
Tax expense			(2)	0
Amounts reclassified from accumulated other comprehensive income, net			5	(3)
Total other comprehensive income (loss) attributable to Aon shareholders			6	(16)
Ending balance	(5)	(14)	(5)	(14)
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(1,861)	(1,333)
Other comprehensive income (loss) before reclassifications, net			(28)	(1,119)
Amounts reclassified from accumulated other comprehensive income
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent			0	0
Tax expense			0	0
Amounts reclassified from accumulated other comprehensive income, net			0	0
Total other comprehensive income (loss) attributable to Aon shareholders			(28)	(1,119)
Ending balance	(1,889)	(2,452)	(1,889)	(2,452)
Postretirement Benefit Obligation
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(2,751)	(2,540)
Other comprehensive income (loss) before reclassifications, net			(26)	20
Amounts reclassified from accumulated other comprehensive income
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent			124	90
Tax expense			(32)	(23)
Amounts reclassified from accumulated other comprehensive income, net			92	67
Total other comprehensive income (loss) attributable to Aon shareholders			66	87
Ending balance	$ (2,685)	$ (2,453)	$ (2,685)	$ (2,453)