Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Consolidating Statement of Income
Commissions, fees and other revenues	$ 3,108	$ 2,726	$ 2,813	$ 2,829	$ 2,980	$ 2,708	$ 2,799	$ 2,748	$ 11,476	$ 11,235	$ 8,457
Fiduciary investment income	7	11	8	12	14	15	12	11	38	52	55
Total revenue	3,115	2,737	2,821	2,841	2,994	2,723	2,811	2,759	11,514	11,287	8,512
Operating income	461	339	394	402	430	328	440	398	1,596	1,596	1,244
Income from continuing operations	307	210	255	249	280	208	265	253	1,021	1,006	759
(Loss) income from discontinued operations			(1)				2	2	(1)	4	(27)
Net income	307	210	254	249	280	208	267	255	1,020	1,010	732
Less: Net income attributable to noncontrolling interests	2	6	8	11	3	10	9	9	27	31	26
Net income attributable to Aon shareholders	$ 305	$ 204	$ 246	$ 238	$ 277	$ 198	$ 258	$ 246	$ 993	$ 979	$ 706
Basic:
lncome from continuing operations (in dollars per share)	$ 0.95	$ 0.62	$ 0.74	$ 0.72	$ 0.84	$ 0.59	$ 0.76	$ 0.72	$ 3.03	$ 2.91	$ 2.50
Loss from discontinued operations (in dollars per share)					$ 0.01					$ 0.01	$ (0.09)
Net income (in dollars per share)	$ 0.95	$ 0.62	$ 0.74	$ 0.72	$ 0.85	$ 0.59	$ 0.76	$ 0.72	$ 3.03	$ 2.92	$ 2.41
Diluted:
Income from continuing operations (in dollars per share)	$ 0.93	$ 0.62	$ 0.73	$ 0.71	$ 0.82	$ 0.59	$ 0.75	$ 0.71	$ 2.99	$ 2.86	$ 2.46
Loss from discontinued operations (in dollars per share)					$ 0.01					$ 0.01	$ (0.09)
Net income (in dollars per share)	$ 0.93	$ 0.62	$ 0.73	$ 0.71	$ 0.83	$ 0.59	$ 0.75	$ 0.71	$ 2.99	$ 2.87	$ 2.37
CLASS A ORDINARY SHARE DATA
Dividends paid per share (in dollars per share)	$ 0.16	$ 0.16	$ 0.16	$ 0.15	$ 0.15	$ 0.15	$ 0.15	$ 0.15	$ 0.62	$ 0.60	$ 0.60
Price range:
High (in dollars per share)	$ 57.92	$ 53.35	$ 52.61	$ 49.51	$ 51.11	$ 52.17	$ 54.58	$ 53.17	$ 57.92	$ 54.58
Low (in dollars per share)	$ 51.78	$ 45.87	$ 45.04	$ 45.58	$ 39.90	$ 39.68	$ 48.66	$ 43.31	$ 45.04	$ 39.68
Shares outstanding (in shares)	310.9	318.7	322.4	326.4	324.4	323.3	326.7	330.5	310.9	324.4
Average monthly trading volume (in shares)	33.7	30.1	47.6	47.0	56.3	55.3	40.2	50.7	39.6	50.6