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Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 467us-gaap_ProfitLoss	$ 315us-gaap_ProfitLoss	$ 313us-gaap_ProfitLoss	$ 336us-gaap_ProfitLoss	$ 360us-gaap_ProfitLoss	$ 264us-gaap_ProfitLoss	$ 252us-gaap_ProfitLoss	$ 272us-gaap_ProfitLoss	$ 1,431us-gaap_ProfitLoss	$ 1,148us-gaap_ProfitLoss	$ 1,020us-gaap_ProfitLoss
Less: Net income attributable to noncontrolling interests	8	6	9	11	5	8	11	11	34	35	27
Net income attributable to Aon shareholders	459	309	304	325	355	256	241	261	1,397	1,113	993
Other comprehensive (loss) gain, net of tax:
Change in fair value of investments									(1)	1	0
Change in fair value of derivatives									5	6	9
Foreign currency translation adjustments									(507)	(65)	109
Post-retirement benefit obligation									(260)	293	(358)
Total other comprehensive (loss) income									(763)	235	(240)
Less: Other comprehensive (loss) income attributable to noncontrolling interests									(3)	(1)	0
Total other comprehensive (loss) income attributable to Aon shareholders									(760)	236	(240)
Comprehensive income attributable to Aon shareholders									$ 637us-gaap_ComprehensiveIncomeNetOfTax	$ 1,349us-gaap_ComprehensiveIncomeNetOfTax	$ 753us-gaap_ComprehensiveIncomeNetOfTax