Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (891)	$ 282	$ 435	$ 1,484
Less: Net income attributable to noncontrolling interests	9	7	43	39
Net income (loss) attributable to Aon shareholders	(900)	275	392	1,445
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	(5)	6	5	4
Foreign currency translation adjustments	(153)	173	(108)	(136)
Postretirement benefit obligation	25	20	87	67
Total other comprehensive income (loss)	(133)	199	(16)	(65)
Less: Other comprehensive income attributable to noncontrolling interests	0	2	0	1
Total other comprehensive income (loss) attributable to Aon shareholders	(133)	197	(16)	(66)
Comprehensive income (loss) attributable to Aon shareholders	$ (1,033)	$ 472	$ 376	$ 1,379