Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Revenue
Commissions, fees and other	$ 2,873	$ 2,786	$ 8,727	$ 8,585
Fiduciary investment income	7	8	19	21
Total revenue	2,880	2,794	8,746	8,606
Expenses
Compensation and benefits	1,707	1,666	5,166	5,103
Other general expenses	756	764	2,249	2,347
Total operating expenses	2,463	2,430	7,415	7,450
Operating income	417	364	1,331	1,156
Interest income	3	3	7	6
Interest expense	(65)	(53)	(188)	(153)
Intercompany interest income (expense)	0	0	0	0
Other income	35	39	34	54
Income before income taxes	390	353	1,184	1,063
Income tax expense (benefit)	75	89	220	275
Income (loss) before equity in earnings of subsidiaries	315	264	964	788
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	315	264	964	788
Less: Net income attributable to noncontrolling interests	6	8	26	30
Net income attributable to Aon shareholders	309	256	938	758
Aon plc [Member]
Revenue
Commissions, fees and other	0	1	0	2
Fiduciary investment income	0	0	0	0
Total revenue	0	1	0	2
Expenses
Compensation and benefits	4	11	29	27
Other general expenses	6	0	18	14
Total operating expenses	10	11	47	41
Operating income	(10)	(10)	(47)	(39)
Interest income	(2)	0	(6)	1
Interest expense	(25)	(6)	(49)	(12)
Intercompany interest income (expense)	113	25	335	11
Other income	2	0	1	0
Income before income taxes	78	9	234	(39)
Income tax expense (benefit)	16	(2)	49	(11)
Income (loss) before equity in earnings of subsidiaries	62	11	185	(28)
Equity in earnings of subsidiaries, net of tax	247	245	753	786
Net income	309	256	938	758
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	309	256	938	758
Aon Corporation [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	5	14	19	34
Other general expenses	14	5	37	29
Total operating expenses	19	19	56	63
Operating income	(19)	(19)	(56)	(63)
Interest income	1	1	1	2
Interest expense	(42)	(39)	(105)	(106)
Intercompany interest income (expense)	(73)	11	(220)	96
Other income	2	13	10	10
Income before income taxes	(131)	(33)	(370)	(61)
Income tax expense (benefit)	(52)	(14)	(144)	(24)
Income (loss) before equity in earnings of subsidiaries	(79)	(19)	(226)	(37)
Equity in earnings of subsidiaries, net of tax	202	127	734	617
Net income	123	108	508	580
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	123	108	508	580
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,873	2,785	8,727	8,583
Fiduciary investment income	7	8	19	21
Total revenue	2,880	2,793	8,746	8,604
Expenses
Compensation and benefits	1,698	1,641	5,118	5,042
Other general expenses	736	759	2,194	2,304
Total operating expenses	2,434	2,400	7,312	7,346
Operating income	446	393	1,434	1,258
Interest income	4	2	12	3
Interest expense	2	(8)	(34)	(35)
Intercompany interest income (expense)	(40)	(36)	(115)	(107)
Other income	31	26	23	44
Income before income taxes	443	377	1,320	1,163
Income tax expense (benefit)	111	105	315	310
Income (loss) before equity in earnings of subsidiaries	332	272	1,005	853
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	332	272	1,005	853
Less: Net income attributable to noncontrolling interests	6	8	26	30
Net income attributable to Aon shareholders	326	264	979	823
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	0	0	0	0
Income before income taxes	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Income (loss) before equity in earnings of subsidiaries	0	0	0	0
Equity in earnings of subsidiaries, net of tax	(449)	(372)	(1,487)	(1,403)
Net income	(449)	(372)	(1,487)	(1,403)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ (449)	$ (372)	$ (1,487)	$ (1,403)