Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) (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
Revenue
Commissions, fees and other	$ 3,292	$ 2,873	$ 2,913	$ 2,941	$ 3,202	$ 2,786	$ 2,891	$ 2,908	$ 12,019	$ 11,787	$ 11,476
Fiduciary investment income	7	7	6	6	7	8	6	7	26	28	38
Total revenue	3,299	2,880	2,919	2,947	3,209	2,794	2,897	2,915	12,045	11,815	11,514
Expenses
Compensation and benefits									7,014	6,945	6,709
Other general expenses									3,065	3,199	3,209
Total operating expenses									10,079	10,144	9,918
Operating income	635	417	445	469	515	364	382	410	1,966	1,671	1,596
Interest income									10	9	10
Interest expense									(255)	(210)	(228)
Intercompany interest income (expense)									0	0	0
Intercompany other income (expense)									0	0	0
Other Income									44	68	2
Income before income taxes									1,765	1,538	1,380
Income tax expense (benefit)									334	390	360
Income (loss) before equity in earnings of subsidiaries									1,431	1,148	1,020
Equity in earnings of subsidiaries, net of tax									0	0	0
Net income	467	315	313	336	360	264	252	272	1,431	1,148	1,020
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
Aon plc [Member]
Revenue
Commissions, fees and other									0	3	2
Fiduciary investment income									0	0	0
Total revenue									0	3	2
Expenses
Compensation and benefits									140	111	40
Other general expenses									3	0	31
Total operating expenses									143	111	71
Operating income									(143)	(108)	(69)
Interest income									(9)	0	0
Interest expense									(75)	(20)	0
Intercompany interest income (expense)									449	120	(19)
Intercompany other income (expense)									342	38	(64)
Other Income									2	0	0
Income before income taxes									566	30	(152)
Income tax expense (benefit)									74	12	(37)
Income (loss) before equity in earnings of subsidiaries									492	18	(115)
Equity in earnings of subsidiaries, net of tax									905	1,095	1,108
Net income									1,397	1,113	993
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									1,397	1,113	993
Aon Corporation [Member]
Revenue
Commissions, fees and other									0	0	1
Fiduciary investment income									0	0	1
Total revenue									0	0	2
Expenses
Compensation and benefits									16	50	50
Other general expenses									5	0	5
Total operating expenses									21	50	55
Operating income									(21)	(50)	(53)
Interest income									2	3	1
Interest expense									(139)	(138)	(164)
Intercompany interest income (expense)									(298)	24	190
Intercompany other income (expense)									(390)	(168)	38
Other Income									5	19	3
Income before income taxes									(841)	(310)	15
Income tax expense (benefit)									(192)	(64)	8
Income (loss) before equity in earnings of subsidiaries									(649)	(246)	7
Equity in earnings of subsidiaries, net of tax									1,214	1,061	932
Net income									565	815	939
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									565	815	939
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other									12,019	11,784	11,474
Fiduciary investment income									26	28	37
Total revenue									12,045	11,812	11,511
Expenses
Compensation and benefits									6,858	6,784	6,619
Other general expenses									3,057	3,199	3,174
Total operating expenses									9,915	9,983	9,793
Operating income									2,130	1,829	1,718
Interest income									17	6	9
Interest expense									(41)	(52)	(64)
Intercompany interest income (expense)									(151)	(144)	(171)
Intercompany other income (expense)									48	130	26
Other Income									37	49	(1)
Income before income taxes									2,040	1,818	1,517
Income tax expense (benefit)									452	442	389
Income (loss) before equity in earnings of subsidiaries									1,588	1,376	1,128
Equity in earnings of subsidiaries, net of tax									0	0	0
Net income									1,588	1,376	1,128
Less: Net income attributable to noncontrolling interests									34	35	27
Net income attributable to Aon shareholders									1,554	1,341	1,101
Consolidating Adjustments [Member]
Revenue
Commissions, fees and other									0	0	(1)
Fiduciary investment income									0	0	0
Total revenue									0	0	(1)
Expenses
Compensation and benefits									0	0	0
Other general expenses									0	0	(1)
Total operating expenses									0	0	(1)
Operating income									0	0	0
Interest income									0	0	0
Interest expense									0	0	0
Intercompany interest income (expense)									0	0	0
Intercompany other income (expense)									0	0	0
Other Income									0	0	0
Income before income 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									(2,119)	(2,156)	(2,040)
Net income									(2,119)	(2,156)	(2,040)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									$ (2,119)	$ (2,156)	$ (2,040)