Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 393	$ 411	$ 1,326	$ 1,202
Less: Net income attributable to noncontrolling interests	14	13	34	32
Net loss attributable to Aon shareholders	379	398	1,292	1,170
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	(1)	3	10	(2)
Foreign currency translation adjustments	115	88	45	(309)
Postretirement benefit obligation	33	23	62	47
Total other comprehensive income (loss)	147	114	117	(264)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	0	1	0	(1)
Total other comprehensive income (loss) attributable to Aon shareholders	147	113	117	(263)
Comprehensive income attributable to Aon shareholders	$ 526	$ 511	$ 1,409	$ 907