Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Condensed Consolidating Statement of Income
Commissions, fees and other revenue	$ 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
Operating income	717	413	277	441	635	417	445	469	1,848	1,966	1,671
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
PER SHARE DATA
Basic net income per share attributable to Aon shareholders (in dollar per share)	$ 2.12	$ 1.05	$ 0.63	$ 1.15	$ 1.60	$ 1.06	$ 1.02	$ 1.07	$ 4.93	$ 4.73
Diluted net income per share attributable to Aon shareholders (in dollars per share)	2.09	1.05	0.62	1.14	1.56	1.04	1.01	1.06	4.88	4.66
CLASS A ORDINARY SHARE DATA
Dividends paid per share (in dollars per share)	0.30	0.30	0.30	0.25	0.25	0.25	0.25	0.18	1.15	0.92	$ 0.68
Price range:
High (in dollars per share)	97.79	103.38	104.70	107.08	98.1	91.28	91.07	87.45	107.08	98.1
Low (in dollars per share)	$ 86.38	$ 87.58	$ 95.32	$ 89.35	$ 78.26	$ 83.06	$ 78.6	$ 76.49	$ 86.38	$ 76.49
Shares outstanding (in shares)	269.8	273.9	279.8	281.7	280.0	285.1	290.5	296.5	269.8	280.0
Average monthly trading volume (in shares)	26.6	26.5	24.9	24.5	34.1	26.3	28.5	32.6	25.6	30.4