Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue
Commissions, fees and other	$ 3,283	$ 2,736	$ 2,800	$ 2,842	$ 3,292	$ 2,873	$ 2,913	$ 2,941	$ 11,661	$ 12,019	$ 11,787
Fiduciary investment income	5	6	5	5	7	7	6	6	21	26	28
Total revenue	3,288	2,742	2,805	2,847	3,299	2,880	2,919	2,947	11,682	12,045	11,815
Expenses
Compensation and benefits									6,837	7,014	6,945
Other general expenses									2,997	3,065	3,199
Total operating expenses									9,834	10,079	10,144
Operating income	717	413	277	441	635	417	445	469	1,848	1,966	1,671
Interest income									14	10	9
Interest expense									(273)	(255)	(210)
Intercompany interest income (expense)									0	0	0
Intercompany other income (expense)									0	0	0
Other income									100	44	68
Income before income taxes									1,689	1,765	1,538
Income tax expense (benefit)									267	334	390
Income (loss) before equity in earnings of subsidiaries									1,422	1,431	1,148
Equity in earnings of subsidiaries, net of tax									0	0	0
Net income	590	303	188	341	467	315	313	336	1,422	1,431	1,148
Less: Net income attributable to noncontrolling interests	6	8	10	13	8	6	9	11	37	34	35
Net income attributable to Aon shareholders	$ 584	$ 295	$ 178	$ 328	$ 459	$ 309	$ 304	$ 325	1,385	1,397	1,113
Aon plc [Member]
Revenue
Commissions, fees and other									0	0	3
Fiduciary investment income									0	0	0
Total revenue									0	0	3
Expenses
Compensation and benefits									136	140	111
Other general expenses									8	3	0
Total operating expenses									144	143	111
Operating income									(144)	(143)	(108)
Interest income									(19)	(9)	0
Interest expense									(121)	(75)	(20)
Intercompany interest income (expense)									429	449	120
Intercompany other income (expense)									302	342	38
Other income									(1)	2	0
Income before income taxes									446	566	30
Income tax expense (benefit)									45	74	12
Income (loss) before equity in earnings of subsidiaries									401	492	18
Equity in earnings of subsidiaries, net of tax									984	905	1,095
Net income									1,385	1,397	1,113
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									1,385	1,397	1,113
Aon Corporation [Member]
Revenue
Commissions, fees and other									0	0	0
Fiduciary investment income									0	0	0
Total revenue									0	0	0
Expenses
Compensation and benefits									32	16	50
Other general expenses									7	5	0
Total operating expenses									39	21	50
Operating income									(39)	(21)	(50)
Interest income									14	2	3
Interest expense									(130)	(139)	(138)
Intercompany interest income (expense)									(479)	(298)	24
Intercompany other income (expense)									(422)	(390)	(168)
Other income									0	5	19
Income before income taxes									(1,056)	(841)	(310)
Income tax expense (benefit)									(262)	(192)	(64)
Income (loss) before equity in earnings of subsidiaries									(794)	(649)	(246)
Equity in earnings of subsidiaries, net of tax									1,319	1,214	1,061
Net income									525	565	815
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									525	565	815
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other									11,661	12,019	11,784
Fiduciary investment income									21	26	28
Total revenue									11,682	12,045	11,812
Expenses
Compensation and benefits									6,669	6,858	6,784
Other general expenses									2,982	3,057	3,199
Total operating expenses									9,651	9,915	9,983
Operating income									2,031	2,130	1,829
Interest income									19	17	6
Interest expense									(22)	(41)	(52)
Intercompany interest income (expense)									50	(151)	(144)
Intercompany other income (expense)									120	48	130
Other income									101	37	49
Income before income taxes									2,299	2,040	1,818
Income tax expense (benefit)									484	452	442
Income (loss) before equity in earnings of subsidiaries									1,815	1,588	1,376
Equity in earnings of subsidiaries, net of tax									525	565	815
Net income									2,340	2,153	2,191
Less: Net income attributable to noncontrolling interests									37	34	35
Net income attributable to Aon shareholders									2,303	2,119	2,156
Consolidating Adjustments [Member]
Revenue
Commissions, fees and other									0	0	0
Fiduciary investment income									0	0	0
Total revenue									0	0	0
Expenses
Compensation and benefits									0	0	0
Other general expenses									0	0	0
Total operating expenses									0	0	0
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									(2,828)	(2,684)	(2,971)
Net income									(2,828)	(2,684)	(2,971)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									$ (2,828)	$ (2,684)	$ (2,971)