Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Income (loss) from continuing operations	$ 411	$ 287	$ 1,202	$ 963
Less: Net income attributable to noncontrolling interests	13	10	32	27
Net income (loss) attributable to Aon shareholders	398	277	1,170	936
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	3	(8)	(2)	(1)
Foreign currency translation adjustments	88	(103)	(309)	30
Postretirement benefit obligation	23	14	47	45
Total other comprehensive income (loss)	114	(97)	(264)	74
Less: Other comprehensive income attributable to noncontrolling interests	1	0	(1)	2
Total other comprehensive income (loss) attributable to Aon shareholders	113	(97)	(263)	72
Comprehensive income attributable to Aon shareholders	$ 511	$ 180	$ 907	$ 1,008