Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 282	$ 228	$ 1,484	$ 1,191
Less: Net income attributable to noncontrolling interests	7	6	39	33
Net income attributable to Aon shareholders	275	222	1,445	1,158
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	6	(2)	4	(3)
Foreign currency translation adjustments	173	(212)	(136)	(182)
Postretirement benefit obligation	20	17	67	62
Total other comprehensive income (loss)	199	(197)	(65)	(123)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	2	(4)	1	(2)
Total other comprehensive income (loss) attributable to Aon shareholders	197	(193)	(66)	(121)
Comprehensive income attributable to Aon shareholders	$ 472	$ 29	$ 1,379	$ 1,037