Guarantee of Registered Securities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Revenues
Commissions, fees and other	$ 2,813	$ 2,799	$ 5,642	$ 5,547
Fiduciary investment income	8	12	20	23
Total revenue	2,821	2,811	5,662	5,570
Expenses
Compensation and benefits	1,639	1,612	3,300	3,209
Other general expenses	788	759	1,566	1,523
Total operating expenses	2,427	2,371	4,866	4,732
Operating income	394	440	796	838
Interest income	2	4	5	10
Interest expense	(57)	(63)	(116)	(126)
Other (expense) income	12	(29)	12	(14)
Income from continuing operations before income taxes	351	352	697	708
Income tax (benefit) expense	96	87	193	190
Income from continuing operations	255	265	504	518
Loss from discontinued operations before income taxes	(1)	1	(1)	5
(Loss) income from discontinued operations	(1)	2	(1)	4
Income taxes		(1)		1
Net income	254	267	503	522
Less: Net income attributable to noncontrolling interests	8	9	19	18
Net income attributable to Aon shareholders	246	258	484	504
Comprehensive income attributable to Aon stockholders
Net (loss) income	254	267	503	522
Less: Net income attributable to noncontrolling interests	8	9	19	18
Net income attributable to Aon stockholders	246	258	484	504
Other comprehensive (loss) income, net of tax:
Change in fair value of derivatives	(17)	(1)	(10)	(5)
Foreign currency translation adjustments	(197)	2	(93)	197
Post-retirement benefit obligation	18	15	39	27
Total other comprehensive (loss) income	(196)	16	(64)	219
Less: Other comprehensive loss attributable to noncontrolling interests	3		2
Total other comprehensive (loss) income attributable to Aon shareholders	(193)	16	(62)	219
Comprehensive income attributable to Aon shareholders	53	274	422	723
Parent Guarantor
Financial Information for Subsidiary Guarantors and Non Guarantors
Parent company's percentage ownership of guarantors	100.00%		100.00%
Expenses
Compensation and benefits	57		57
Other general expenses	4		3
Total operating expenses	61		60
Operating income	(61)		(60)
Intercompany interest (expense) income	(3)		(3)
Income from continuing operations before income taxes	(64)		(63)
Income tax (benefit) expense	(16)		(16)
Income from continuing operations	(48)		(47)
Equity in earnings of subsidiaries, net of tax	294	258	531	504
Net income	246	258	484	504
Net income attributable to Aon shareholders	246	258	484	504
Comprehensive income attributable to Aon stockholders
Net (loss) income	246	258	484	504
Net income attributable to Aon stockholders	246	258	484	504
Other comprehensive (loss) income, net of tax:
Equity in other comprehensive (loss) income of subsidiary, net of tax	(193)	16	(62)	219
Total other comprehensive (loss) income attributable to Aon shareholders	(193)	16	(62)	219
Comprehensive income attributable to Aon shareholders	53	274	422	723
Subsidiary Issuer
Revenues
Fiduciary investment income			1	1
Total revenue			1	1
Expenses
Compensation and benefits		39	66	100
Other general expenses	7		22
Total operating expenses	7	39	88	100
Operating income	(7)	(39)	(87)	(99)
Interest income		2		4
Interest expense	(40)	(39)	(80)	(91)
Intercompany interest (expense) income	47	44	93	88
Other (expense) income	(5)	(25)	3	(27)
Income from continuing operations before income taxes	(5)	(57)	(71)	(125)
Income tax (benefit) expense	(2)	(21)	(27)	(46)
Income from continuing operations	(3)	(36)	(44)	(79)
Loss from discontinued operations before income taxes		1		5
(Loss) income from discontinued operations		2		4
Income taxes		(1)		1
Equity in earnings of subsidiaries, net of tax	268	263	517	510
Net income	265	229	473	435
Net income attributable to Aon shareholders	265	229	473	435
Comprehensive income attributable to Aon stockholders
Net (loss) income	265	229	473	435
Net income attributable to Aon stockholders	265	229	473	435
Other comprehensive (loss) income, net of tax:
Change in fair value of derivatives			1	(1)
Foreign currency translation adjustments	(1)	(7)		(9)
Post-retirement benefit obligation	5	2	7	3
Total other comprehensive (loss) income	4	(5)	8	(7)
Equity in other comprehensive (loss) income of subsidiary, net of tax	(197)	21	(70)	223
Total other comprehensive (loss) income attributable to Aon shareholders	(193)	16	(62)	216
Comprehensive income attributable to Aon shareholders	72	245	411	651
Other non-guarantor subsidiaries
Revenues
Commissions, fees and other	2,813	2,799	5,642	5,547
Fiduciary investment income	8	12	19	22
Total revenue	2,821	2,811	5,661	5,569
Expenses
Compensation and benefits	1,582	1,573	3,177	3,109
Other general expenses	777	759	1,541	1,523
Total operating expenses	2,359	2,332	4,718	4,632
Operating income	462	479	943	937
Interest income	2	2	5	6
Interest expense	(17)	(24)	(36)	(35)
Intercompany interest (expense) income	(44)	(44)	(90)	(88)
Other (expense) income	17	(4)	9	13
Income from continuing operations before income taxes	420	409	831	833
Income tax (benefit) expense	114	108	236	236
Income from continuing operations	306	301	595	597
Loss from discontinued operations before income taxes	(1)		(1)
(Loss) income from discontinued operations	(1)		(1)
Equity in earnings of subsidiaries, net of tax	265	229	473	435
Net income	570	530	1,067	1,032
Less: Net income attributable to noncontrolling interests	8	9	19	18
Net income attributable to Aon shareholders	562	521	1,048	1,014
Comprehensive income attributable to Aon stockholders
Net (loss) income	570	530	1,067	1,032
Less: Net income attributable to noncontrolling interests	8	9	19	18
Net income attributable to Aon stockholders	562	521	1,048	1,014
Other comprehensive (loss) income, net of tax:
Change in fair value of derivatives	(17)	(1)	(11)	(4)
Foreign currency translation adjustments	(196)	9	(93)	206
Post-retirement benefit obligation	13	13	32	24
Total other comprehensive (loss) income	(200)	21	(72)	226
Equity in other comprehensive (loss) income of subsidiary, net of tax	(193)	16	(62)	216
Less: Other comprehensive loss attributable to noncontrolling interests	(3)		(2)
Total other comprehensive (loss) income attributable to Aon shareholders	(390)	37	(132)	442
Comprehensive income attributable to Aon shareholders	172	558	916	1,456
Consolidating adjustments
Expenses
Equity in earnings of subsidiaries, net of tax	(827)	(750)	(1,521)	(1,449)
Net income	(827)	(750)	(1,521)	(1,449)
Net income attributable to Aon shareholders	(827)	(750)	(1,521)	(1,449)
Comprehensive income attributable to Aon stockholders
Net (loss) income	(827)	(750)	(1,521)	(1,449)
Net income attributable to Aon stockholders	(827)	(750)	(1,521)	(1,449)
Other comprehensive (loss) income, net of tax:
Equity in other comprehensive (loss) income of subsidiary, net of tax	583	(53)	194	(658)
Total other comprehensive (loss) income attributable to Aon shareholders	583	(53)	194	(658)
Comprehensive income attributable to Aon shareholders	$ (244)	$ (803)	$ (1,327)	$ (2,107)
7.375% Debt securities due December 2012
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	7.375%		7.375%
3.50% senior notes due September 2015
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	3.50%		3.50%
3.125% Senior notes due 2016
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	3.125%		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%		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%		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%		6.25%