Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Revenue
Commissions, fees and other	$ 2,891	$ 2,813	$ 5,799	$ 5,642
Fiduciary investment income	6	8	13	20
Total revenue	2,897	2,821	5,812	5,662
Expenses
Compensation and benefits	1,712	1,639	3,437	3,300
Other general expenses	803	788	1,583	1,566
Total operating expenses	2,515	2,427	5,020	4,866
Operating income	382	394	792	796
Interest income	2	2	3	5
Interest expense	(48)	(57)	(100)	(116)
Intercompany interest (expense) income	0	0	0	0
Other income	6	12	15	12
Income from continuing operations before income taxes	342	351	710	697
Income tax (benefit) expense	90	96	186	193
Income from continuing operations	252	255	524	504
Loss from discontinued operations before income taxes	0	(1)	0	(1)
Income taxes	0	0	0	0
Loss from discontinued operations	0	(1)	0	(1)
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	252	254	524	503
Less: Net income attributable to noncontrolling interests	11	8	22	19
Net income attributable to Aon shareholders	241	246	502	484
Aon plc [Member]
Revenue
Commissions, fees and other	1	0	2	0
Fiduciary investment income	0	0	0	0
Total revenue	1	0	2	0
Expenses
Compensation and benefits	4	2	15	2
Other general expenses	9	10	16	10
Total operating expenses	13	12	31	12
Operating income	(12)	(12)	(29)	(12)
Interest income	0	0	0	0
Interest expense	(4)	0	(6)	0
Intercompany interest (expense) income	(7)	(3)	(14)	(3)
Other income	0	0	0	0
Income from continuing operations before income taxes	(23)	(15)	(49)	(15)
Income tax (benefit) expense	(3)	(1)	(9)	(1)
Income from continuing operations	(20)	(14)	(40)	(14)
Loss from discontinued operations before income taxes	0	0	0	0
Income taxes	0	0	0	0
Loss from discontinued operations	0	0	0	0
Equity in earnings of subsidiaries, net of tax	261	260	542	498
Net income	241	246	502	484
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	241	246	502	484
Aon Corporation [Member]
Revenue
Commissions, fees and other	0	0	0	0
Fiduciary investment income	0	0	0	1
Total revenue	0	0	0	1
Expenses
Compensation and benefits	5	0	21	67
Other general expenses	17	7	23	21
Total operating expenses	22	7	44	88
Operating income	(22)	(7)	(44)	(87)
Interest income	1	0	1	0
Interest expense	(33)	(40)	(67)	(80)
Intercompany interest (expense) income	43	47	86	93
Other income	(2)	(5)	(3)	3
Income from continuing operations before income taxes	(13)	(5)	(27)	(71)
Income tax (benefit) expense	(6)	(2)	(11)	(27)
Income from continuing operations	(7)	(3)	(16)	(44)
Loss from discontinued operations before income taxes	0	0	0	0
Income taxes	0	0	0	0
Loss from discontinued operations	0	0	0	0
Equity in earnings of subsidiaries, net of tax	246	234	509	484
Net income	239	231	493	440
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	239	231	493	440
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,890	2,813	5,797	5,642
Fiduciary investment income	6	8	13	19
Total revenue	2,896	2,821	5,810	5,661
Expenses
Compensation and benefits	1,703	1,637	3,401	3,231
Other general expenses	777	771	1,544	1,535
Total operating expenses	2,480	2,408	4,945	4,766
Operating income	416	413	865	895
Interest income	1	2	2	5
Interest expense	(11)	(17)	(27)	(36)
Intercompany interest (expense) income	(36)	(44)	(72)	(90)
Other income	8	17	18	9
Income from continuing operations before income taxes	378	371	786	783
Income tax (benefit) expense	99	99	206	221
Income from continuing operations	279	272	580	562
Loss from discontinued operations before income taxes	0	(1)	0	(1)
Income taxes	0	0	0	0
Loss from discontinued operations	0	(1)	0	(1)
Equity in earnings of subsidiaries, net of tax	239	231	493	440
Net income	518	502	1,073	1,001
Less: Net income attributable to noncontrolling interests	11	8	22	19
Net income attributable to Aon shareholders	507	494	1,051	982
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 (expense) income	0	0	0	0
Other income	0	0	0	0
Income from continuing operations before income taxes	0	0	0	0
Income tax (benefit) expense	0	0	0	0
Income from continuing operations	0	0	0	0
Loss from discontinued operations before income taxes	0	0	0	0
Income taxes	0	0	0	0
Loss from discontinued operations	0	0	0	0
Equity in earnings of subsidiaries, net of tax	(746)	(725)	(1,544)	(1,422)
Net income	(746)	(725)	(1,544)	(1,422)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(746)	(725)	(1,544)	(1,422)
Pro Forma [Member]
Expenses
Income from continuing operations	252	255	524	504
Loss from discontinued operations	0	(1)	0	(1)
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	252	254	524	503
Less: Net income attributable to noncontrolling interests	11	8	22	19
Net income attributable to Aon shareholders	241	246	502	484
Pro Forma [Member] | Aon plc [Member]
Expenses
Income from continuing operations	(20)	(14)	(40)	(14)
Loss from discontinued operations	0	0	0	0
Equity in earnings of subsidiaries, net of tax	261	260	542	498
Net income	241	246	502	484
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	241	246	502	484
Pro Forma [Member] | Aon Corporation [Member]
Expenses
Income from continuing operations	(7)	(3)	(16)	(44)
Loss from discontinued operations	0	0	0	0
Equity in earnings of subsidiaries, net of tax	246	234	509	484
Net income	239	231	493	440
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	239	231	493	440
Pro Forma [Member] | Other Non-Guarantor Subsidiaries [Member]
Expenses
Income from continuing operations	279	272	580	562
Loss from discontinued operations	0	(1)	0	(1)
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	279	271	580	561
Less: Net income attributable to noncontrolling interests	11	8	22	19
Net income attributable to Aon shareholders	268	263	558	542
Pro Forma [Member] | Consolidating Adjustments [Member]
Expenses
Income from continuing operations	0	0	0	0
Loss from discontinued operations	0	0	0	0
Equity in earnings of subsidiaries, net of tax	(507)	(494)	(1,051)	(982)
Net income	(507)	(494)	(1,051)	(982)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ (507)	$ (494)	$ (1,051)	$ (982)