Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 228	$ 153	$ 1,191	$ 821
Less: Net income attributable to noncontrolling interests	6	6	33	32
Net income attributable to Aon shareholders	222	147	1,158	789
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	(2)	1	(3)	14
Foreign currency translation adjustments	(212)	(50)	(182)	(263)
Postretirement benefit obligation	17	(62)	62	108
Total other comprehensive loss	(197)	(111)	(123)	(141)
Less: Other comprehensive loss attributable to noncontrolling interests	(4)	(3)	(2)	(6)
Total other comprehensive loss attributable to Aon shareholders	(193)	(108)	(121)	(135)
Comprehensive income (loss) attributable to Aon shareholders	$ 29	$ 39	$ 1,037	$ 654