Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 610	$ 305
Less: Net income attributable to noncontrolling interests	16	14
Net income attributable to Aon shareholders	594	291
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	14	(2)
Foreign currency translation adjustments	247	147
Postretirement benefit obligation	48	18
Total other comprehensive income	309	163
Less: Other comprehensive income attributable to noncontrolling interests	3	1
Total other comprehensive income attributable to Aon shareholders	306	162
Comprehensive income (loss) attributable to Aon shareholders	$ 900	$ 453