Shareholders' Equity (Deficit) - Components of Accumulated other comprehensive loss, net of related tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ (429)	$ 1,158	$ 3,583
Other comprehensive income (loss) before reclassifications	74	(1,112)	(63)
Tax benefit	52	153	(56)
Other comprehensive income (loss) before reclassifications, net	126	(959)	(119)
Amounts reclassified from accumulated other comprehensive income(loss)	167	280	144
Tax expense	(43)	(73)	(35)
Amounts reclassified from accumulated other comprehensive income (loss), net	124	207	109
Total other comprehensive income (loss) attributable to Aon shareholders	250	(752)	(10)
Ending Balance	(742)	(429)	1,158
Change in Fair Value of Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(11)	2	1
Other comprehensive income (loss) before reclassifications	8	(15)	0
Tax benefit	(1)	4	0
Other comprehensive income (loss) before reclassifications, net	7	(11)	0
Amounts reclassified from accumulated other comprehensive income(loss)	8	(2)	2
Tax expense	(2)	0	(1)
Amounts reclassified from accumulated other comprehensive income (loss), net	6	(2)	1
Total other comprehensive income (loss) attributable to Aon shareholders	13	(13)	1
Ending Balance	2	(11)	2
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(1,861)	(1,333)	(1,045)
Other comprehensive income (loss) before reclassifications	278	(528)	(290)
Tax benefit	(1)	0	2
Other comprehensive income (loss) before reclassifications, net	277	(528)	(288)
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	277	(528)	(288)
Ending Balance	(1,584)	(1,861)	(1,333)
Post-retirement Benefit Obligations
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(2,751)	(2,540)	(2,817)
Other comprehensive income (loss) before reclassifications	(212)	(569)	227
Tax benefit	54	149	(58)
Other comprehensive income (loss) before reclassifications, net	(158)	(420)	169
Amounts reclassified from accumulated other comprehensive income(loss)	159	282	142
Tax expense	(41)	(73)	(34)
Amounts reclassified from accumulated other comprehensive income (loss), net	118	209	108
Total other comprehensive income (loss) attributable to Aon shareholders	(40)	(211)	277
Ending Balance	(2,791)	(2,751)	(2,540)
AOCI attributable to parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(4,623)	(3,871)	(3,861)
Ending Balance	$ (4,373)	$ (4,623)	$ (3,871)