Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 933	$ 791
Less: Net income attributable to noncontrolling interests	20	19
Net income (loss) attributable to Aon shareholders	913	772
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments	11	(5)
Foreign currency translation adjustments	(70)	(397)
Postretirement benefit obligation	29	24
Total other comprehensive income (loss)	(30)	(378)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	0	(2)
Total other comprehensive income (loss) attributable to Aon shareholders	(30)	(376)
Comprehensive income attributable to Aon shareholders	$ 883	$ 396