Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 1,308	$ 2,018	$ 1,573
Less: Net income attributable to noncontrolling interests	53	49	41
Net income attributable to Aon shareholders	1,255	1,969	1,532
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	1	13	3
Foreign currency translation adjustments	(289)	263	14
Postretirement benefit obligation	277	(101)	(141)
Total other comprehensive income (loss)	(11)	175	(124)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	(1)	3	0
Total other comprehensive income (loss) attributable to Aon shareholders	(10)	172	(124)
Comprehensive income attributable to Aon shareholders	$ 1,245	$ 2,141	$ 1,408