Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 2,018	$ 1,573	$ 1,174
Less: Net income attributable to noncontrolling interests	49	41	40
Net income attributable to Aon shareholders	1,969	1,532	1,134
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	13	3	11
Foreign currency translation adjustments	263	14	(444)
Postretirement benefit obligation	(101)	(141)	17
Total other comprehensive income (loss)	175	(124)	(416)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	3	0	(4)
Total other comprehensive income (loss) attributable to Aon shareholders	172	(124)	(412)
Comprehensive income attributable to Aon shareholders	$ 2,141	$ 1,408	$ 722