Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 305	$ 337
Less: Net income attributable to noncontrolling interests	14	12
Net income attributable to Aon shareholders	291	325
Other comprehensive (loss) income, net of tax:
Change in fair value of financial instruments	(2)	(7)
Foreign currency translation adjustments	147	(79)
Postretirement benefit obligation	18	(201)
Total other comprehensive income (loss)	163	(287)
Less: Other comprehensive income attributable to noncontrolling interests	1	0
Total other comprehensive income (loss) attributable to Aon shareholders	162	(287)
Comprehensive income (loss) attributable to Aon shareholders	$ 453	$ 38