Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 2,770	$ 2,349	$ 2,561	$ 3,090	$ 2,909	$ 2,340	$ 2,368	$ 2,381	$ 10,770	$ 9,998	$ 9,409
Operating income	499	262	(16)	799	601	256	(127)	335	1,544	1,065	1,811
Net income from continuing operations	284	155	57	604	17	196	(43)	265	1,100	435	1,253
Net income from discontinued operations	69	(2)	1	6	(29)	(4)	821	40	74	828	177
Net income	353	153	58	610	(12)	192	778	305	1,174	1,263	1,430
Less: Net income attributable to noncontrolling interests	8	6	10	16	7	7	9	14	40	37	34
Net income attributable to Aon shareholders	$ 345	$ 147	$ 48	$ 594	$ (19)	$ 185	$ 769	$ 291	$ 1,134	$ 1,226	$ 1,396
Basic net income per share attributable to Aon shareholders
Basic net income per share attributable to Aon shareholders, continuing operations (in dollars per share)	$ 1.14	$ 0.61	$ 0.19	$ 2.37	$ 0.04	$ 0.74	$ (0.20)	$ 0.95	$ 4.32	$ 1.54	$ 4.55
Basic net income per share attributable to Aon shareholders, discontinued operations (in dollars per share)	0.28	(0.01)	0.01	0.02	(0.12)	(0.02)	3.13	0.15	0.30	3.20	0.66
Basic net income per share attributable to Aon shareholders (in dollars per share)	1.42	0.60	0.20	2.39	(0.08)	0.72	2.93	1.10	4.62	4.74	5.21
Diluted net income per share attributable to Aon shareholders
Diluted net income per share attributable to Aon shareholders, continuing operations (in dollars per share)	1.13	0.61	0.19	2.35	0.04	0.73	(0.20)	0.94	4.29	1.53	4.51
Diluted net income per share attributable to Aon shareholders, discontinued operations (in dollars per share)	0.28	(0.01)	0.00	0.02	(0.11)	(0.01)	3.13	0.15	0.30	3.17	0.65
Diluted net income per share attributable to Aon shareholders (in dollars per share)	$ 1.41	$ 0.60	$ 0.19	$ 2.37	$ (0.07)	$ 0.72	$ 2.93	$ 1.09	$ 4.59	$ 4.70	$ 5.16