Condensed Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 252	$ 254	$ 524	$ 503
Less: Net income attributable to noncontrolling interests	11	8	22	19
Net income attributable to Aon shareholders	241	246	502	484
Other comprehensive (loss) income, net of tax:
Change in fair value of investments	13	0	13	0
Change in fair value of derivatives	(7)	(17)	(21)	(10)
Foreign currency translation adjustments	(58)	(197)	(232)	(93)
Post-retirement benefit obligation	18	18	41	39
Total other comprehensive loss	(34)	(196)	(199)	(64)
Less: Other comprehensive loss attributable to noncontrolling interests	(2)	(3)	(2)	(2)
Total other comprehensive loss attributable to Aon shareholders	(32)	(193)	(197)	(62)
Comprehensive income attributable to Aon shareholders	$ 209	$ 53	$ 305	$ 422