Guarantee of Registered Securities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Revenue
Commissions, fees and other	$ 2,908	$ 2,829
Fiduciary investment income	7	12
Total revenue	2,915	2,841
Expenses
Compensation and benefits	1,725	1,661
Other general expenses	780	778
Total operating expenses	2,505	2,439
Operating income	410	402
Interest income	1	3
Interest expense	(52)	(59)
Intercompany interest (expense) income	0	0
Other income	9	0
Income before income taxes	368	346
Income tax (benefit) expense	96	97
Net income	272	249
Equity in earnings of subsidiaries, net of tax	0	0
Net income	272	249
Less: Net income attributable to noncontrolling interests	11	11
Net income attributable to Aon shareholders	261	238
Comprehensive income attributable to Aon stockholders
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
Equity in other comprehensive loss of subsidiaries, net of tax	0	0
Less: Other comprehensive loss 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
Aon plc
Financial Information for Subsidiary Guarantors and Non Guarantors
Parent company's percentage ownership of guarantors	100.00%
Revenue
Commissions, fees and other	1	0
Fiduciary investment income	0	0
Total revenue	1	0
Expenses
Compensation and benefits	11	0
Other general expenses	7	0
Total operating expenses	18	0
Operating income	(17)	0
Interest income	0	0
Interest expense	(2)	0
Intercompany interest (expense) income	(7)	0
Other income	0	0
Income before income taxes	(26)	0
Income tax (benefit) expense	(5)	0
Net income	(21)	0
Equity in earnings of subsidiaries, net of tax	282	238
Net income	261	238
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	261	238
Comprehensive income attributable to Aon stockholders
Net income	261	238
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	261	238
Other comprehensive (loss) income, net of tax:
Change in fair value of derivatives	0	0
Foreign currency translation adjustments	0	0
Post-retirement benefit obligation	0	0
Total other comprehensive (loss) income	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	(165)	131
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	(165)	131
Comprehensive income attributable to Aon shareholders	96	369
Aon Corporation
Revenue
Commissions, fees and other	2	0
Fiduciary investment income	0	1
Total revenue	2	1
Expenses
Compensation and benefits	15	69
Other general expenses	8	8
Total operating expenses	23	77
Operating income	(21)	(76)
Interest income	1	0
Interest expense	(34)	(40)
Intercompany interest (expense) income	43	46
Other income	(1)	8
Income before income taxes	(12)	(62)
Income tax (benefit) expense	(5)	(24)
Net income	(7)	(38)
Equity in earnings of subsidiaries, net of tax	262	247
Net income	255	209
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	255	209
Comprehensive income attributable to Aon stockholders
Net income	255	209
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	255	209
Other comprehensive (loss) income, net of tax:
Change in fair value of derivatives	1	0
Foreign currency translation adjustments	(8)	4
Post-retirement benefit obligation	7	6
Total other comprehensive (loss) income	0	10
Equity in other comprehensive loss of subsidiaries, net of tax	(169)	117
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	(169)	127
Comprehensive income attributable to Aon shareholders	86	336
Other Non-Guarantor Subsidiaries
Revenue
Commissions, fees and other	2,907	2,829
Fiduciary investment income	7	11
Total revenue	2,914	2,840
Expenses
Compensation and benefits	1,699	1,592
Other general expenses	767	770
Total operating expenses	2,466	2,362
Operating income	448	478
Interest income	0	3
Interest expense	(16)	(19)
Intercompany interest (expense) income	(36)	(46)
Other income	10	(8)
Income before income taxes	406	408
Income tax (benefit) expense	106	121
Net income	300	287
Equity in earnings of subsidiaries, net of tax	255	209
Net income	555	496
Less: Net income attributable to noncontrolling interests	11	11
Net income attributable to Aon shareholders	544	485
Comprehensive income attributable to Aon stockholders
Net income	555	496
Less: Net income attributable to noncontrolling interests	11	11
Net income attributable to Aon shareholders	544	485
Other comprehensive (loss) income, net of tax:
Change in fair value of derivatives	(15)	7
Foreign currency translation adjustments	(166)	100
Post-retirement benefit obligation	16	15
Total other comprehensive (loss) income	(165)	122
Equity in other comprehensive loss of subsidiaries, net of tax	(169)	127
Less: Other comprehensive loss attributable to noncontrolling interests	0	1
Total other comprehensive (loss) income attributable to Aon shareholders	(334)	248
Comprehensive income attributable to Aon shareholders	210	733
Consolidating Adjustments
Revenue
Commissions, fees and other	(2)	0
Fiduciary investment income	0	0
Total revenue	(2)	0
Expenses
Compensation and benefits	0	0
Other general expenses	(2)	0
Total operating expenses	(2)	0
Operating income	0	0
Interest income	0	0
Interest expense	0	0
Intercompany interest (expense) income	0	0
Other income	0	0
Income before income taxes	0	0
Income tax (benefit) expense	0	0
Net income	0	0
Equity in earnings of subsidiaries, net of tax	(799)	(694)
Net income	(799)	(694)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	(799)	(694)
Comprehensive income attributable to Aon stockholders
Net income	(799)	(694)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	(799)	(694)
Other comprehensive (loss) income, net of tax:
Change in fair value of derivatives	0	0
Foreign currency translation adjustments	0	0
Post-retirement benefit obligation	0	0
Total other comprehensive (loss) income	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	503	(375)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	503	(375)
Comprehensive income attributable to Aon shareholders	$ (296)	$ (1,069)
3.50% senior notes due September 2015
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	3.50%
3.125% Senior notes due 2016
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	3.125%
5.00% Senior notes due September 2020
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	5.00%
8.205% Junior subordinated deferrable interest debentures due January 2027
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	8.205%
6.25% Senior notes due September 2040
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	6.25%
4.250% Senior notes due 2042
Financial Information for Subsidiary Guarantors and Non Guarantors
Stated interest rate	4.25%