Shareholders' Equity - Components of Accumulated other comprehensive loss, net of related tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 3,583	$ 3,449	$ 4,219
Other comprehensive income (loss) before reclassifications	(63)	4	(282)
Tax benefit (expense)	(56)	62	58
Other comprehensive income (loss) before reclassifications, net	(119)	66	(224)
Amounts reclassified from accumulated other comprehensive income	144	140	123
Tax expense	(35)	(34)	(23)
Amounts reclassified from accumulated other comprehensive income (loss), net	109	106	100
Total other comprehensive income (loss) attributable to Aon shareholders	(10)	172	(124)
Ending Balance	1,158	3,583	3,449
Change in Fair Value of Investments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	1	(12)	(15)
Other comprehensive income (loss) before reclassifications	0	1	(10)
Tax benefit (expense)	0	0	1
Other comprehensive income (loss) before reclassifications, net	0	1	(9)
Amounts reclassified from accumulated other comprehensive income	2	15	14
Tax expense	(1)	(3)	(2)
Amounts reclassified from accumulated other comprehensive income (loss), net	1	12	12
Total other comprehensive income (loss) attributable to Aon shareholders	1	13	3
Ending Balance	2	1	(12)
Foreign Currency Translation Adjustments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(1,045)	(1,305)	(1,319)
Other comprehensive income (loss) before reclassifications	(290)	258	15
Tax benefit (expense)	2	2	(1)
Other comprehensive income (loss) before reclassifications, net	(288)	260	14
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	(288)	260	14
Ending Balance	(1,333)	(1,045)	(1,305)
Post-retirement Benefit Obligations
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,817)	(2,716)	(2,575)
Other comprehensive income (loss) before reclassifications	227	(255)	(287)
Tax benefit (expense)	(58)	60	58
Other comprehensive income (loss) before reclassifications, net	169	(195)	(229)
Amounts reclassified from accumulated other comprehensive income	142	125	109
Tax expense	(34)	(31)	(21)
Amounts reclassified from accumulated other comprehensive income (loss), net	108	94	88
Total other comprehensive income (loss) attributable to Aon shareholders	277	(101)	(141)
Ending Balance	(2,540)	(2,817)	(2,716)
AOCI attributable to parent
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(3,861)	(4,033)	(3,909)
Ending Balance	$ (3,871)	$ (3,861)	$ (4,033)