Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenue
Commissions, fees and other	$ 2,786	$ 2,726	$ 8,585	$ 8,368
Fiduciary investment income	8	11	21	31
Total revenue	2,794	2,737	8,606	8,399
Expenses
Compensation and benefits	1,666	1,620	5,103	4,920
Other general expenses	764	778	2,347	2,344
Total operating expenses	2,430	2,398	7,450	7,264
Operating income	364	339	1,156	1,135
Interest income	3	1	6	6
Interest expense	(53)	(57)	(153)	(173)
Intercompany interest income (expense)	0	0	0	0
Other income (expense)	39	(9)	54	3
Income from continuing operations before income taxes	353	274	1,063	971
Income tax (benefit) expense	89	64	275	257
Income from continuing operations	264	210	788	714
Loss from discontinued operations before income taxes	0	0	0	(1)
Income taxes	0	0	0	0
Loss from discontinued operations	0	0	0	(1)
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	264	210	788	713
Less: Net income attributable to noncontrolling interests	8	6	30	25
Net income attributable to Aon shareholders	256	204	758	688
Aon plc [Member]
Revenue
Commissions, fees and other	1	0	2	0
Fiduciary investment income	0	0	0	0
Total revenue	1	0	2	0
Expenses
Compensation and benefits	11	4	27	0
Other general expenses	0	19	14	20
Total operating expenses	11	23	41	20
Operating income	(10)	(23)	(39)	(20)
Interest income	0	0	1	0
Interest expense	(6)	0	(12)	0
Intercompany interest income (expense)	25	(8)	11	(11)
Other income (expense)	0	0	0	0
Income from continuing operations before income taxes	9	(31)	(39)	(31)
Income tax (benefit) expense	(2)	(3)	(11)	(4)
Income from continuing operations	11	(28)	(28)	(27)
Loss from discontinued operations before income taxes	0	0	0	0
Income taxes	0	0	0	0
Loss from discontinued operations	0	0	0	0
Equity in earnings of subsidiaries, net of tax	245	232	786	715
Net income	256	204	758	688
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	256	204	758	688
Aon Corporation [Member]
Revenue
Commissions, fees and other	0	0	0	0
Fiduciary investment income	0	0	0	1
Total revenue	0	0	0	1
Expenses
Compensation and benefits	14	(5)	34	31
Other general expenses	5	6	29	27
Total operating expenses	19	1	63	58
Operating income	(19)	(1)	(63)	(57)
Interest income	1	0	2	1
Interest expense	(39)	(45)	(106)	(125)
Intercompany interest income (expense)	11	48	96	141
Other income (expense)	13	10	10	13
Income from continuing operations before income taxes	(33)	12	(61)	(27)
Income tax (benefit) expense	(14)	16	(24)	(11)
Income from continuing operations	(19)	(4)	(37)	(16)
Loss from discontinued operations before income taxes	0	0	0	0
Income taxes	0	0	0	0
Loss from discontinued operations	0	0	0	0
Equity in earnings of subsidiaries, net of tax	127	186	617	598
Net income	108	182	580	582
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	108	182	580	582
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,785	2,726	8,583	8,368
Fiduciary investment income	8	11	21	30
Total revenue	2,793	2,737	8,604	8,398
Expenses
Compensation and benefits	1,641	1,621	5,042	4,889
Other general expenses	759	753	2,304	2,297
Total operating expenses	2,400	2,374	7,346	7,186
Operating income	393	363	1,258	1,212
Interest income	2	1	3	5
Interest expense	(8)	(12)	(35)	(48)
Intercompany interest income (expense)	(36)	(40)	(107)	(130)
Other income (expense)	26	(19)	44	(10)
Income from continuing operations before income taxes	377	293	1,163	1,029
Income tax (benefit) expense	105	51	310	272
Income from continuing operations	272	242	853	757
Loss from discontinued operations before income taxes	0	0	0	(1)
Income taxes	0	0	0	0
Loss from discontinued operations	0	0	0	(1)
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	272	242	853	756
Less: Net income attributable to noncontrolling interests	8	6	30	25
Net income attributable to Aon shareholders	264	236	823	731
Consolidating Adjustments [Member]
Revenue
Commissions, fees and other	0	0	0	0
Fiduciary investment income	0	0	0	0
Total revenue	0	0	0	0
Expenses
Compensation and benefits	0	0	0	0
Other general expenses	0	0	0	0
Total operating expenses	0	0	0	0
Operating income	0	0	0	0
Interest income	0	0	0	0
Interest expense	0	0	0	0
Intercompany interest income (expense)	0	0	0	0
Other income (expense)	0	0	0	0
Income from continuing operations before income taxes	0	0	0	0
Income tax (benefit) expense	0	0	0	0
Income from continuing operations	0	0	0	0
Loss from discontinued operations before income taxes	0	0	0	0
Income taxes	0	0	0	0
Loss from discontinued operations	0	0	0	0
Equity in earnings of subsidiaries, net of tax	(372)	(418)	(1,403)	(1,313)
Net income	(372)	(418)	(1,403)	(1,313)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ (372)	$ (418)	$ (1,403)	$ (1,313)