Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Consolidating Statement of Income
Commissions, fees and other revenue	$ 3,202	$ 2,786	$ 2,891	$ 2,908	$ 3,108	$ 2,726	$ 2,813	$ 2,829	$ 11,787	$ 11,476	$ 11,235
Fiduciary investment income	7	8	6	7	7	11	8	12	28	38	52
Total revenue	3,209	2,794	2,897	2,915	3,115	2,737	2,821	2,841	11,815	11,514	11,287
Operating income	515	364	382	410	461	339	394	402	1,671	1,596	1,596
Net income	360	264	252	272	307	210	254	249	1,148	1,020	1,010
Less: Net income attributable to noncontrolling interests	5	8	11	11	2	6	8	11	35	27	31
Net income attributable to Aon shareholders	$ 355	$ 256	$ 241	$ 261	$ 305	$ 204	$ 246	$ 238	$ 1,113	$ 993	$ 979
PER SHARE DATA
Basic net income per share attributable to Aon shareholders	$ 1.16	$ 0.83	$ 0.77	$ 0.82	$ 0.95	$ 0.62	$ 0.74	$ 0.72	$ 3.57	$ 3.02
Diluted net income per share attributable to Aon shareholders	$ 1.14	$ 0.82	$ 0.76	$ 0.82	$ 0.93	$ 0.62	$ 0.73	$ 0.71	$ 3.53	$ 2.99
CLASS A ORDINARY SHARE DATA
Dividends paid per share (in dollars per share)	$ 0.18	$ 0.18	$ 0.18	$ 0.16	$ 0.16	$ 0.16	$ 0.16	$ 0.15	$ 0.68	$ 0.62	$ 0.60
Price range:
High (in dollars per share)	$ 84.33	$ 76.3	$ 67.26	$ 61.87	$ 57.92	$ 53.35	$ 52.61	$ 49.51	$ 84.33	$ 57.92
Low (in dollars per share)	$ 70.72	$ 64.2	$ 58.48	$ 54.65	$ 51.78	$ 45.87	$ 45.04	$ 45.58	$ 54.65	$ 45.04
Shares outstanding (in shares)	300.7	301.0	307.5	309.1	310.9	318.7	322.4	326.4	300.7	310.9
Average monthly trading volume (in shares)	37.6	30.9	34.9	46.4	33.7	30.1	47.6	47.0	37.5	39.6