Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 287	$ 58	$ 963	$ 668
Less: Net income attributable to noncontrolling interests	10	10	27	26
Net income attributable to Aon shareholders	277	48	936	642
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	(8)	(1)	(1)	13
Foreign currency translation adjustments	(103)	(460)	30	(213)
Postretirement benefit obligation	14	122	45	170
Total other comprehensive income (loss)	(97)	(339)	74	(30)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	0	(6)	2	(3)
Total other comprehensive income (loss) attributable to Aon shareholders	(97)	(333)	72	(27)
Comprehensive income (loss) attributable to Aon shareholders	$ 180	$ (285)	$ 1,008	$ 615