Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 58	$ 778	$ 668	$ 1,083
Less: Net income attributable to noncontrolling interests	10	9	26	23
Net income attributable to Aon shareholders	48	769	642	1,060
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	(1)	4	13	2
Foreign currency translation adjustments	(460)	44	(213)	191
Postretirement benefit obligation	122	20	170	38
Total other comprehensive income (loss)	(339)	68	(30)	231
Less: Other comprehensive loss attributable to noncontrolling interests	(6)	(5)	(3)	(4)
Total other comprehensive income (loss) attributable to Aon shareholders	(333)	73	(27)	235
Comprehensive income (loss) attributable to Aon shareholders	$ (285)	$ 842	$ 615	$ 1,295