Condensed Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Statement of Other Comprehensive Income [Abstract]
Net income	$ 272	$ 249
Less: Net income attributable to noncontrolling interests	11	11
Net income attributable to Aon shareholders	261	238
Other comprehensive (loss) income, net of tax:
Change in fair value of derivatives	(14)	7
Foreign currency translation adjustments	(174)	104
Post-retirement benefit obligation	23	21
Total other comprehensive (loss) income	(165)	132
Less: Other comprehensive income attributable to noncontrolling interests	0	1
Total other comprehensive (loss) income attributable to Aon shareholders	(165)	131
Comprehensive income attributable to Aon shareholders	$ 96	$ 369