Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 2,909	$ 2,340	$ 2,368	$ 2,381	$ 2,650	$ 2,201	$ 2,282	$ 2,276	$ 9,998	$ 9,409	$ 9,480
Operating income	489	265	(118)	343	463	368	387	420	979	1,638	1,587
Net income from continuing operations	17	196	(43)	265	384	284	273	312	435	1,253	1,253
Income from discontinued operations, net of tax	(29)	(4)	821	40	75	42	35	25	828	177	169
Net income	(12)	192	778	305	459	326	308	337	1,263	1,430	1,422
Less: Net income attributable to noncontrolling interests	7	7	9	14	7	7	8	12	37	34	37
Net income attributable to Aon shareholders	$ (19)	$ 185	$ 769	$ 291	$ 452	$ 319	$ 300	$ 325	$ 1,226	$ 1,396	$ 1,385
Basic net income per share attributable to Aon shareholders
Basic net income per share attributable to Aon shareholders, continuing operations (in dollars per share)	$ 0.04	$ 0.74	$ (0.20)	$ 0.95	$ 1.42	$ 1.03	$ 0.99	$ 1.11	$ 1.54	$ 4.55	$ 4.33
Basic net income per share attributable to Aon shareholders, discontinued operations (in dollars per share)	(0.12)	(0.02)	3.13	0.15	0.28	0.16	0.13	0.09	3.20	0.66	0.60
Basic net income per share attributable to Aon shareholders (in dollars per share)	(0.08)	0.72	2.93	1.10	1.70	1.19	1.12	1.20	4.74	5.21	4.93
Diluted net income per share attributable to Aon shareholders
Diluted net income per share attributable to Aon shareholders, continuing operations (in dollars per share)	0.04	0.73	(0.20)	0.94	1.40	1.03	0.98	1.10	1.53	4.51	4.28
Diluted net income per share attributable to Aon shareholders, discontinued operations (in dollars per share)	(0.11)	(0.01)	3.13	0.15	0.28	0.15	0.13	0.09	3.17	0.65	0.60
Diluted net income per share attributable to Aon shareholders (in dollars per share)	$ (0.07)	$ 0.72	$ 2.93	$ 1.09	$ 1.68	$ 1.18	$ 1.11	$ 1.19	$ 4.70	$ 5.16	$ 4.88