Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Statement of Comprehensive Income [Abstract]
Net income	$ 360	$ 264	$ 252	$ 272	$ 307	$ 210	$ 254	$ 249	$ 1,148	$ 1,020	$ 1,010
Less: Net income attributable to noncontrolling interests	5	8	11	11	2	6	8	11	35	27	31
Net income attributable to Aon shareholders	355	256	241	261	305	204	246	238	1,113	993	979
Other comprehensive gain (loss), net of tax:
Change in fair value of investments									1	0	0
Change in fair value of derivatives									6	9	(13)
Foreign currency translation adjustments									(65)	109	(43)
Post-retirement benefit obligation									293	(358)	(396)
Total other comprehensive gain (loss)									235	(240)	(452)
Less: Other comprehensive (loss) income attributable to noncontrolling interests									(1)	0	1
Total other comprehensive gain (loss) attributable to Aon shareholders									236	(240)	(453)
Comprehensive income attributable to Aon shareholders									$ 1,349	$ 753	$ 526