Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 676	$ 610
Less: Net income attributable to noncontrolling interests	17	16
Net income attributable to Aon shareholders	659	594
Other comprehensive income, net of tax:
Change in fair value of financial instruments	7	14
Foreign currency translation adjustments	133	247
Postretirement benefit obligation	31	48
Total other comprehensive income	171	309
Less: Other comprehensive income attributable to noncontrolling interests	2	3
Total other comprehensive income attributable to Aon shareholders	169	306
Comprehensive income (loss) attributable to Aon shareholders	$ 828	$ 900