Guarantee of Registered Securities Condensed Consolidating Statement of Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenue
Commissions, fees and other	$ 3,202	$ 2,786	$ 2,891	$ 2,908	$ 3,108	$ 2,726	$ 2,813	$ 2,829	$ 11,787	$ 11,476	$ 11,235
Fiduciary investment income	7	8	6	7	7	11	8	12	28	38	52
Total revenue	3,209	2,794	2,897	2,915	3,115	2,737	2,821	2,841	11,815	11,514	11,287
Expenses
Compensation and benefits									6,945	6,709	6,567
Other general expenses									3,199	3,209	3,124
Total operating expenses									10,144	9,918	9,691
Operating income	515	364	382	410	461	339	394	402	1,671	1,596	1,596
Interest income									9	10	18
Interest expense									(210)	(228)	(245)
Intercompany interest (expense) income									0	0	0
Other Income									68	2	19
Income (loss) before taxes									1,538	1,380	1,388
Income tax expense (benefit)									390	360	378
(Income) loss before equity in earnings of subsidiaries									1,148	1,020	1,010
Equity in earnings of subsidiaries, net of tax									0	0	0
Net income	360	264	252	272	307	210	254	249	1,148	1,020	1,010
Less: Net income attributable to noncontrolling interests	5	8	11	11	2	6	8	11	35	27	31
Net income attributable to Aon shareholders	355	256	241	261	305	204	246	238	1,113	993	979
Aon plc [Member]
Revenue
Commissions, fees and other									3	2	0
Fiduciary investment income									0	0	0
Total revenue									3	2	0
Expenses
Compensation and benefits									69	117	0
Other general expenses									4	18	0
Total operating expenses									73	135	0
Operating income									(70)	(133)	0
Interest income									0	0	0
Interest expense									(20)	0	0
Intercompany interest (expense) income									120	(19)	0
Other Income									0	0	0
Income (loss) before taxes									30	(152)	0
Income tax expense (benefit)									12	(37)	0
(Income) loss before equity in earnings of subsidiaries									18	(115)	0
Equity in earnings of subsidiaries, net of tax									1,095	1,108	979
Net income									1,113	993	979
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									1,113	993	979
Aon Corporation [Member]
Revenue
Commissions, fees and other									0	1	0
Fiduciary investment income									0	1	2
Total revenue									0	2	2
Expenses
Compensation and benefits									183	24	118
Other general expenses									35	47	47
Total operating expenses									218	71	165
Operating income									(218)	(69)	(163)
Interest income									3	1	4
Interest expense									(138)	(164)	(171)
Intercompany interest (expense) income									24	190	180
Other Income									19	57	36
Income (loss) before taxes									(310)	15	(114)
Income tax expense (benefit)									(64)	8	(43)
(Income) loss before equity in earnings of subsidiaries									(246)	7	(71)
Equity in earnings of subsidiaries, net of tax									1,061	932	868
Net income									815	939	797
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									815	939	797
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other									11,784	11,474	11,236
Fiduciary investment income									28	37	50
Total revenue									11,812	11,511	11,286
Expenses
Compensation and benefits									6,693	6,568	6,449
Other general expenses									3,160	3,145	3,078
Total operating expenses									9,853	9,713	9,527
Operating income									1,959	1,798	1,759
Interest income									6	9	14
Interest expense									(52)	(64)	(74)
Intercompany interest (expense) income									(144)	(171)	(180)
Other Income									49	(55)	(17)
Income (loss) before taxes									1,818	1,517	1,502
Income tax expense (benefit)									442	389	421
(Income) loss before equity in earnings of subsidiaries									1,376	1,128	1,081
Equity in earnings of subsidiaries, net of tax									0	0	0
Net income									1,376	1,128	1,081
Less: Net income attributable to noncontrolling interests									35	27	31
Net income attributable to Aon shareholders									1,341	1,101	1,050
Consolidating Adjustments [Member]
Revenue
Commissions, fees and other									0	(1)	(1)
Fiduciary investment income									0	0	0
Total revenue									0	(1)	(1)
Expenses
Compensation and benefits									0	0	0
Other general expenses									0	(1)	(1)
Total operating expenses									0	(1)	(1)
Operating income									0	0	0
Interest income									0	0	0
Interest expense									0	0	0
Intercompany interest (expense) income									0	0	0
Other Income									0	0	0
Income (loss) before taxes									0	0	0
Income tax expense (benefit)									0	0	0
(Income) loss before equity in earnings of subsidiaries									0	0	0
Equity in earnings of subsidiaries, net of tax									(2,156)	(2,040)	(1,847)
Net income									(2,156)	(2,040)	(1,847)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									$ (2,156)	$ (2,040)	$ (1,847)