Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenue
Commissions, fees and other	$ 2,736	$ 2,873	$ 8,378	$ 8,727
Fiduciary investment income	6	7	16	19
Total revenue	2,742	2,880	8,394	8,746
Expenses
Compensation and benefits	1,644	1,707	4,980	5,166
Other general expenses	685	756	2,283	2,249
Total operating expenses	2,329	2,463	7,263	7,415
Operating income	413	417	1,131	1,331
Interest income	3	3	10	7
Interest expense	(72)	(65)	(205)	(188)
Intercompany interest income (expense)	0	0	0	0
Intercompany other (expense) income	0	0	0	0
Other income	8	35	51	34
Income before income taxes	352	390	987	1,184
Income tax (benefit) expense	49	75	155	220
(Loss) income before equity in earnings of subsidiaries	303	315	832	964
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	303	315	832	964
Less: Net income attributable to noncontrolling interests	8	6	31	26
Net income attributable to Aon shareholders	295	309	801	938
Aon plc [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	28	29	85	100
Other general expenses	1	1	10	7
Total operating expenses	29	30	95	107
Operating income	(29)	(30)	(95)	(107)
Interest income	(5)	(2)	(14)	(6)
Interest expense	(33)	(25)	(87)	(49)
Intercompany interest income (expense)	119	113	358	335
Intercompany other (expense) income	(57)	(29)	(162)	(101)
Other income	0	2	0	1
Income before income taxes	(5)	29	0	73
Income tax (benefit) expense	(1)	6	0	15
(Loss) income before equity in earnings of subsidiaries	(4)	23	0	58
Equity in earnings of subsidiaries, net of tax	299	286	801	880
Net income	295	309	801	938
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	295	309	801	938
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	8	4	29	17
Other general expenses	3	1	6	6
Total operating expenses	11	5	35	23
Operating income	(11)	(5)	(35)	(23)
Interest income	4	1	10	1
Interest expense	(34)	(42)	(101)	(105)
Intercompany interest income (expense)	(128)	(73)	(349)	(220)
Intercompany other (expense) income	(17)	(14)	(40)	(33)
Other income	2	2	10	10
Income before income taxes	(184)	(131)	(505)	(370)
Income tax (benefit) expense	(63)	(52)	(180)	(144)
(Loss) income before equity in earnings of subsidiaries	(121)	(79)	(325)	(226)
Equity in earnings of subsidiaries, net of tax	355	241	925	861
Net income	234	162	600	635
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	234	162	600	635
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,736	2,873	8,378	8,727
Fiduciary investment income	6	7	16	19
Total revenue	2,742	2,880	8,394	8,746
Expenses
Compensation and benefits	1,608	1,674	4,866	5,049
Other general expenses	681	754	2,267	2,236
Total operating expenses	2,289	2,428	7,133	7,285
Operating income	453	452	1,261	1,461
Interest income	4	4	14	12
Interest expense	(5)	2	(17)	(34)
Intercompany interest income (expense)	9	(40)	(9)	(115)
Intercompany other (expense) income	74	43	202	134
Other income	6	31	41	23
Income before income taxes	541	492	1,492	1,481
Income tax (benefit) expense	113	121	335	349
(Loss) income before equity in earnings of subsidiaries	428	371	1,157	1,132
Equity in earnings of subsidiaries, net of tax	234	162	600	635
Net income	662	533	1,757	1,767
Less: Net income attributable to noncontrolling interests	8	6	31	26
Net income attributable to Aon shareholders	654	527	1,726	1,741
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
Intercompany other (expense) income	0	0	0	0
Other income	0	0	0	0
Income before income taxes	0	0	0	0
Income tax (benefit) expense	0	0	0	0
(Loss) income before equity in earnings of subsidiaries	0	0	0	0
Equity in earnings of subsidiaries, net of tax	(888)	(689)	(2,326)	(2,376)
Net income	(888)	(689)	(2,326)	(2,376)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ (888)	$ (689)	$ (2,326)	$ (2,376)