Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 153	$ 192	$ 821	$ 1,275
Less: Net income attributable to noncontrolling interests	6	7	32	30
Net income attributable to Aon shareholders	147	185	789	1,245
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	1	11	14	13
Foreign currency translation adjustments	(50)	243	(263)	434
Postretirement benefit obligation	(62)	18	108	56
Total other comprehensive income (loss)	(111)	272	(141)	503
Less: Other comprehensive income (loss) attributable to noncontrolling interests	(3)	7	(6)	3
Total other comprehensive income (loss) attributable to Aon shareholders	(108)	265	(135)	500
Comprehensive income (loss) attributable to Aon shareholders	$ 39	$ 450	$ 654	$ 1,745