Shareholders' Equity (Deficit) - Components of Accumulated other comprehensive loss, net of related tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 1,158	$ 3,583	$ 3,443
Other comprehensive income (loss) before reclassifications	(1,112)	(63)	4
Tax benefit	153	(56)	62
Other comprehensive income (loss) before reclassifications, net	(959)	(119)	66
Amounts reclassified from accumulated other comprehensive income	280	144	140
Tax expense	(73)	(35)	(34)
Amounts reclassified from accumulated other comprehensive income (loss), net	207	109	106
Total other comprehensive income (loss) attributable to Aon shareholders	(752)	(10)	172
Ending Balance	(429)	1,158	3,583
Change in Fair Value of Investments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	2	1	(12)
Other comprehensive income (loss) before reclassifications	(15)	0	1
Tax benefit	4	0	0
Other comprehensive income (loss) before reclassifications, net	(11)	0	1
Amounts reclassified from accumulated other comprehensive income	(2)	2	15
Tax expense	0	(1)	(3)
Amounts reclassified from accumulated other comprehensive income (loss), net	(2)	1	12
Total other comprehensive income (loss) attributable to Aon shareholders	(13)	1	13
Ending Balance	(11)	2	1
Foreign Currency Translation Adjustments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(1,333)	(1,045)	(1,305)
Other comprehensive income (loss) before reclassifications	(528)	(290)	258
Tax benefit	0	2	2
Other comprehensive income (loss) before reclassifications, net	(528)	(288)	260
Amounts reclassified from accumulated other comprehensive income	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	(528)	(288)	260
Ending Balance	(1,861)	(1,333)	(1,045)
Post-retirement Benefit Obligations
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,540)	(2,817)	(2,716)
Other comprehensive income (loss) before reclassifications	(569)	227	(255)
Tax benefit	149	(58)	60
Other comprehensive income (loss) before reclassifications, net	(420)	169	(195)
Amounts reclassified from accumulated other comprehensive income	282	142	125
Tax expense	(73)	(34)	(31)
Amounts reclassified from accumulated other comprehensive income (loss), net	209	108	94
Total other comprehensive income (loss) attributable to Aon shareholders	(211)	277	(101)
Ending Balance	(2,751)	(2,540)	(2,817)
AOCI attributable to parent
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(3,871)	(3,861)	(4,033)
Ending Balance	$ (4,623)	$ (3,871)	$ (3,861)