Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidated Statements of Comprehensive Income
Net income	$ 307	$ 210	$ 254	$ 249	$ 280	$ 208	$ 267	$ 255	$ 1,020	$ 1,010	$ 732
Less: Net income attributable to noncontrolling interests	2	6	8	11	3	10	9	9	27	31	26
Net income attributable to Aon shareholders	305	204	246	238	277	198	258	246	993	979	706
Other comprehensive gain (loss), net of tax:
Change in accounting principle											(44)
Change in fair value of derivatives									9	(13)	(24)
Foreign currency translation adjustments									109	(43)	(135)
Post-retirement benefit obligation									(358)	(396)	(41)
Total other comprehensive loss									(240)	(452)	(244)
Less: Other comprehensive income (loss) attributable to noncontrolling interests										1	(2)
Total other comprehensive loss attributable to Aon shareholders									(240)	(453)	(242)
Comprehensive income attributable to Aon shareholders									$ 753	$ 526	$ 464