Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 314	$ 303	$ 921	$ 832
Less: Net income attributable to noncontrolling interests	7	8	27	31
Net income attributable to Aon shareholders	307	295	894	801
Other comprehensive (loss) income, net of tax:
Change in fair value of financial instruments	0	(10)	(11)	(11)
Foreign currency translation adjustments	(89)	(229)	(227)	(376)
Post-retirement benefit obligation	18	18	(132)	62
Total other comprehensive loss	(71)	(221)	(370)	(325)
Less: Other comprehensive loss attributable to noncontrolling interests	0	(4)	0	(6)
Total other comprehensive loss attributable to Aon shareholders	(71)	(217)	(370)	(319)
Comprehensive income (loss) attributable to Aon shareholders	$ 236	$ 78	$ 524	$ 482