Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 791	$ 676
Less: Net income attributable to noncontrolling interests	19	17
Net income attributable to Aon shareholders	772	659
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	(5)	7
Foreign currency translation adjustments	(397)	133
Postretirement benefit obligation	24	31
Total other comprehensive income (loss)	(378)	171
Less: Other comprehensive income attributable to noncontrolling interests	(2)	2
Total other comprehensive income (loss) attributable to Aon shareholders	(376)	169
Comprehensive income (loss) attributable to Aon shareholders	$ 396	$ 828