Shareholders' Equity (Deficit) - Components of Accumulated Other Comprehensive Loss, Net of Related Tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ (742)	$ (429)	$ 1,158
Other comprehensive loss before reclassifications	(490)	74	(1,112)
Tax benefit (expense)	13	52	153
Other comprehensive income (loss) before reclassifications, net	(477)	126	(959)
Amounts reclassified from accumulated other comprehensive income (loss)	141	167	280
Tax expense	(36)	(43)	(73)
Amounts reclassified from accumulated other comprehensive income (loss), net	105	124	207
Total other comprehensive income (loss) attributable to Aon shareholders	(372)	250	(752)
Ending Balance	6,305	(742)	(429)
AOCI attributable to parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(4,373)	(4,623)	(3,871)
Ending Balance	(4,745)	(4,373)	(4,623)
Change in Fair Value of Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	2	(11)	2
Other comprehensive loss before reclassifications	99	8	(15)
Tax benefit (expense)	(26)	(1)	4
Other comprehensive income (loss) before reclassifications, net	73	7	(11)
Amounts reclassified from accumulated other comprehensive income (loss)	(2)	8	(2)
Tax expense	1	(2)	0
Amounts reclassified from accumulated other comprehensive income (loss), net	(1)	6	(2)
Total other comprehensive income (loss) attributable to Aon shareholders	72	13	(13)
Ending Balance	74	2	(11)
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(1,584)	(1,861)	(1,333)
Other comprehensive loss before reclassifications	(479)	278	(528)
Tax benefit (expense)	12	(1)	0
Other comprehensive income (loss) before reclassifications, net	(467)	277	(528)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0
Tax expense	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss), net	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(467)	277	(528)
Ending Balance	(2,051)	(1,584)	(1,861)
Post-retirement Benefit Obligations
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(2,791)	(2,751)	(2,540)
Other comprehensive loss before reclassifications	(110)	(212)	(569)
Tax benefit (expense)	27	54	149
Other comprehensive income (loss) before reclassifications, net	(83)	(158)	(420)
Amounts reclassified from accumulated other comprehensive income (loss)	143	159	282
Tax expense	(37)	(41)	(73)
Amounts reclassified from accumulated other comprehensive income (loss), net	106	118	209
Total other comprehensive income (loss) attributable to Aon shareholders	23	(40)	(211)
Ending Balance	$ (2,768)	$ (2,791)	$ (2,751)