Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Revenue
Commissions, fees and other	$ 2,800	$ 2,913	$ 5,642	$ 5,854
Fiduciary investment income	5	6	10	12
Total revenue	2,805	2,919	5,652	5,866
Expenses
Compensation and benefits	1,653	1,708	3,336	3,459
Other general expenses	875	766	1,598	1,493
Total operating expenses	2,528	2,474	4,934	4,952
Operating income	277	445	718	914
Interest income	4	2	7	4
Interest expense	(68)	(65)	(133)	(123)
Intercompany interest income (expense)	0	0	0	0
Intercompany other (expense) income	0	0	0	0
Other income (expense)	1	(2)	43	(1)
Income before income taxes	214	380	635	794
Income tax (benefit) expense	26	67	106	145
(Loss) income before equity in earnings of subsidiaries	188	313	529	649
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	188	313	529	649
Less: Net income attributable to noncontrolling interests	10	9	23	20
Net income attributable to Aon shareholders	178	304	506	629
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	5	6	57	72
Other general expenses	(4)	1	9	6
Total operating expenses	1	7	66	78
Operating income	(1)	(7)	(66)	(78)
Interest income	(5)	(2)	(9)	(4)
Interest expense	(29)	(15)	(54)	(25)
Intercompany interest income (expense)	119	111	239	222
Intercompany other (expense) income	(56)	(54)	(106)	(72)
Other income (expense)	(1)	(1)	1	(1)
Income before income taxes	27	32	5	42
Income tax (benefit) expense	6	6	1	9
(Loss) income before equity in earnings of subsidiaries	21	26	4	33
Equity in earnings of subsidiaries, net of tax	157	278	502	596
Net income	178	304	506	629
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	178	304	506	629
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	10	6	21	13
Other general expenses	2	3	4	5
Total operating expenses	12	9	25	18
Operating income	(12)	(9)	(25)	(18)
Interest income	4	0	7	1
Interest expense	(33)	(32)	(68)	(63)
Intercompany interest income (expense)	(116)	(73)	(221)	(147)
Intercompany other (expense) income	(15)	(4)	(23)	(19)
Other income (expense)	0	8	8	8
Income before income taxes	(172)	(110)	(322)	(238)
Income tax (benefit) expense	(66)	(42)	(117)	(92)
(Loss) income before equity in earnings of subsidiaries	(106)	(68)	(205)	(146)
Equity in earnings of subsidiaries, net of tax	170	288	571	621
Net income	64	220	366	475
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	64	220	366	475
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,800	2,913	5,642	5,854
Fiduciary investment income	5	6	10	12
Total revenue	2,805	2,919	5,652	5,866
Expenses
Compensation and benefits	1,638	1,696	3,258	3,374
Other general expenses	877	762	1,585	1,482
Total operating expenses	2,515	2,458	4,843	4,856
Operating income	290	461	809	1,010
Interest income	5	4	9	7
Interest expense	(6)	(18)	(11)	(35)
Intercompany interest income (expense)	(3)	(38)	(18)	(75)
Intercompany other (expense) income	71	58	129	91
Other income (expense)	2	(9)	34	(8)
Income before income taxes	359	458	952	990
Income tax (benefit) expense	86	103	222	228
(Loss) income before equity in earnings of subsidiaries	273	355	730	762
Equity in earnings of subsidiaries, net of tax	64	220	366	475
Net income	337	575	1,096	1,237
Less: Net income attributable to noncontrolling interests	10	9	23	20
Net income attributable to Aon shareholders	327	566	1,073	1,217
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 (expense)	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	(391)	(786)	(1,439)	(1,692)
Net income	(391)	(786)	(1,439)	(1,692)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ (391)	$ (786)	$ (1,439)	$ (1,692)