Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Consolidating Statement of Income
Commissions, fees and other revenue	$ 3,292	$ 2,873	$ 2,913	$ 2,941	$ 3,202	$ 2,786	$ 2,891	$ 2,908	$ 12,019	$ 11,787	$ 11,476
Fiduciary investment income	7	7	6	6	7	8	6	7	26	28	38
Total revenue	3,299	2,880	2,919	2,947	3,209	2,794	2,897	2,915	12,045	11,815	11,514
Operating income	635	417	445	469	515	364	382	410	1,966	1,671	1,596
Net income	467	315	313	336	360	264	252	272	1,431	1,148	1,020
Less: Net income attributable to noncontrolling interests	8	6	9	11	5	8	11	11	34	35	27
Net income attributable to Aon shareholders	$ 459	$ 309	$ 304	$ 325	$ 355	$ 256	$ 241	$ 261	$ 1,397	$ 1,113	$ 993
Basic net income per share attributable to Aon shareholders (in dollars per share)	$ 1.60	$ 1.06	$ 1.02	$ 1.07					$ 4.73	$ 3.57	$ 3.02
PER SHARE DATA
Basic net income per share attributable to Aon shareholders					$ 1.16	$ 0.83	$ 0.77	$ 0.82		$ 3.57
Diluted net income per share attributable to Aon shareholders (in dollars per share)	$ 1.56	$ 1.04	$ 1.01	$ 1.06					$ 4.66	$ 3.53	$ 2.99
Diluted net income per share attributable to Aon shareholders					$ 1.14	$ 0.82	$ 0.76	$ 0.82		$ 3.53
CLASS A ORDINARY SHARE DATA
Dividends paid per share (in dollars per share)	$ 0.25	$ 0.25	$ 0.25	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.16	$ 0.92	$ 0.68	$ 0.62
Price range:
High (in dollars per share)	$ 98.1	$ 91.28	$ 91.07	$ 87.45	$ 84.33	$ 76.3	$ 67.26	$ 61.87	$ 98.1	$ 84.33
Low (in dollars per share)	$ 78.26	$ 83.06	$ 78.6	$ 76.49	$ 70.72	$ 64.2	$ 58.48	$ 54.65	$ 76.49	$ 54.65
Shares outstanding (in shares)	280.0	285.1	290.5	296.5	300.7	301.0	307.5	309.1	280.0	300.7
Average monthly trading volume (in shares)	34.1	26.3	28.5	32.6	37.6	30.9	34.9	46.4	30.4	37.5