Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Commissions, fees and other revenue	$ 3,256	$ 2,754	$ 2,795	$ 2,800	$ 3,222	$ 2,746	$ 2,831	$ 2,862			$ 11,605	$ 11,661	$ 12,019
Fiduciary investment income	6	6	5	5	5	6	5	5			22	21	26
Total revenue	3,262	2,760	2,800	2,805	3,227	2,752	2,836	2,867			11,627	11,682	12,045
Operating income	568	436	439	463	656	423	308	461			1,906	1,848	1,966
Net income	459	326	308	337	540	311	214	357	$ 645	$ 971	1,430	1,422	1,431
Less: Net income attributable to noncontrolling interests	7	7	8	12	6	8	10	13			34	37	34
Net income attributable to Aon shareholders	$ 452	$ 319	$ 300	$ 325	$ 534	$ 303	$ 204	$ 344			$ 1,396	$ 1,385	$ 1,397
PER SHARE DATA
Basic net income per share attributable to Aon shareholders (in dollars per share)	$ 1.70	$ 1.19	$ 1.12	$ 1.20	$ 1.94	$ 1.08	$ 0.72	$ 1.21			$ 5.21	$ 4.93
Diluted net income per share attributable to Aon shareholders (in dollars per share)	$ 1.68	$ 1.18	$ 1.11	$ 1.19	$ 1.91	$ 1.07	$ 0.71	$ 1.20			$ 5.16	$ 4.88
Scenario, Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Commissions, fees and other revenue	$ 3,317	$ 2,740	$ 2,761	$ 2,787	$ 3,283	$ 2,736	$ 2,800	$ 2,842			$ 11,605	$ 11,661
Fiduciary investment income	6	6	5	5	5	6	5	5			22	21
Total revenue	3,323	2,746	2,766	2,792	3,288	2,742	2,805	2,847			11,627	11,682
Operating income	629	422	405	450	717	413	277	441			1,906	1,848
Net income	509	314	280	327	590	303	188	341	$ 607	$ 921	1,430	1,422
Less: Net income attributable to noncontrolling interests	7	7	8	12	6	8	10	13			34	37
Net income attributable to Aon shareholders	$ 502	$ 307	$ 272	$ 315	$ 584	$ 295	$ 178	$ 328			$ 1,396	$ 1,385
PER SHARE DATA
Basic net income per share attributable to Aon shareholders (in dollars per share)	$ 1.89	$ 1.15	$ 1.01	$ 1.16	$ 2.12	$ 1.05	$ 0.63	$ 1.15			$ 5.21	$ 4.93
Diluted net income per share attributable to Aon shareholders (in dollars per share)	$ 1.87	$ 1.14	$ 1.01	$ 1.15	$ 2.09	$ 1.04	$ 0.62	$ 1.14			$ 5.16	$ 4.88
Restatement Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Commissions, fees and other revenue	$ (61)	$ 14	$ 34	$ 13	$ (61)	$ 10	$ 31	$ 20			$ 0	$ 0
Fiduciary investment income	0	0	0	0	0	0	0	0			0	0
Total revenue	(61)	14	34	13	(61)	10	31	20			0	0
Operating income	(61)	14	34	13	(61)	10	31	20			0	0
Net income	(50)	12	28	10	(50)	8	26	16			0	0
Less: Net income attributable to noncontrolling interests	0	0	0	0	0	0	0	0			0	0
Net income attributable to Aon shareholders	$ (50)	$ 12	$ 28	$ 10	$ (50)	$ 8	$ 26	$ 16			$ 0	$ 0
PER SHARE DATA
Basic net income per share attributable to Aon shareholders (in dollars per share)	$ (0.19)	$ 0.04	$ 0.11	$ 0.04	$ (0.18)	$ 0.03	$ 0.09	$ 0.06			$ 0.00	$ 0.00
Diluted net income per share attributable to Aon shareholders (in dollars per share)	$ (0.19)	$ 0.04	$ 0.10	$ 0.04	$ (0.18)	$ 0.03	$ 0.09	$ 0.06			$ 0.00	$ 0.00