Condensed Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 264	$ 210	$ 788	$ 713
Less: Net income attributable to noncontrolling interests	8	6	30	25
Net income attributable to Aon shareholders	256	204	758	688
Other comprehensive (loss) income, net of tax:
Change in fair value of investments	(14)	0	(1)	0
Change in fair value of derivatives	9	15	(12)	5
Foreign currency translation adjustments	155	164	(77)	71
Post-retirement benefit obligation	24	7	65	46
Total other comprehensive income (loss)	174	186	(25)	122
Less: Other comprehensive income attributable to noncontrolling interests	3	4	1	2
Total other comprehensive income (loss) attributable to Aon shareholders	171	182	(26)	120
Comprehensive income attributable to Aon shareholders	$ 427	$ 386	$ 732	$ 808