Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ 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
Revenue
Total revenue	$ 2,770	$ 2,349	$ 2,561	$ 3,090	$ 2,909	$ 2,340	$ 2,368	$ 2,381	$ 10,770	$ 9,998	$ 9,409
Expenses
Compensation and benefits									6,103	6,003	5,514
Information technology									484	419	386
Premises									370	348	343
Depreciation of fixed assets									176	187	162
Amortization and impairment of intangible assets									593	704	157
Other general expenses									1,500	1,272	1,036
Total operating expenses									9,226	8,933	7,598
Operating income	499	262	(16)	799	601	256	(127)	335	1,544	1,065	1,811
Interest income									5	27	9
Interest expense									(278)	(282)	(282)
Intercompany interest income (expense)									0	0	0
Intercompany other income (expense)									0	0	0
Other income (expense)									(25)	(125)	(137)
Income from continuing operations before income taxes									1,246	685	1,401
Income tax expense (benefit)									146	250	148
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 (loss) before equity in earnings of subsidiaries									1,174	1,263	1,430
Equity in earnings of subsidiaries									0	0	0
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
Aon plc
Revenue
Total revenue									0	0	0
Expenses
Compensation and benefits									74	150	130
Information technology									0	0	0
Premises									0	0	0
Depreciation of fixed assets									0	0	0
Amortization and impairment of intangible assets									0	0
Other general expenses									4	12	0
Total operating expenses									78	162	130
Operating income									(78)	(162)	(130)
Interest income									0	0	0
Interest expense									(203)	(202)	(196)
Intercompany interest income (expense)									15	14	14
Intercompany other income (expense)									97	247	274
Other income (expense)									9	(27)	15
Income from continuing operations before income taxes									(160)	(130)	(23)
Income tax expense (benefit)									(60)	(43)	(55)
Net income from continuing operations									(100)	(87)	32
Net income from discontinued operations									0	0	0
Net income (loss) before equity in earnings of subsidiaries									(100)	(87)	32
Equity in earnings of subsidiaries									1,255	1,295	1,382
Net income									1,155	1,208	1,414
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									1,155	1,208	1,414
Aon Corporation
Revenue
Total revenue									0	0	0
Expenses
Compensation and benefits									(1)	26	6
Information technology									0	0	0
Premises									0	0	0
Depreciation of fixed assets									0	0	0
Amortization and impairment of intangible assets									0	0	0
Other general expenses									64	(6)	2
Total operating expenses									63	20	8
Operating income									(63)	(20)	(8)
Interest income									58	52	16
Interest expense									(101)	(94)	(101)
Intercompany interest income (expense)									(514)	(543)	(541)
Intercompany other income (expense)									(399)	(411)	(361)
Other income (expense)									(48)	12	(170)
Income from continuing operations before income taxes									(1,067)	(1,004)	(1,165)
Income tax expense (benefit)									(192)	(110)	(325)
Net income from continuing operations									(875)	(894)	(840)
Net income from discontinued operations									0	0	0
Net income (loss) before equity in earnings of subsidiaries									(875)	(894)	(840)
Equity in earnings of subsidiaries									1,004	1,121	1,249
Net income									129	227	409
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									129	227	409
Other Non-Guarantor Subsidiaries
Revenue
Total revenue									10,770	9,998	9,409
Expenses
Compensation and benefits									6,030	5,827	5,378
Information technology									484	419	386
Premises									370	348	343
Depreciation of fixed assets									176	187	162
Amortization and impairment of intangible assets									593	704	157
Other general expenses									1,432	1,266	1,034
Total operating expenses									9,085	8,751	7,460
Operating income									1,685	1,247	1,949
Interest income									0	4	22
Interest expense									(27)	(15)	(14)
Intercompany interest income (expense)									499	529	527
Intercompany other income (expense)									302	164	87
Other income (expense)									35	(128)	36
Income from continuing operations before income taxes									2,494	1,801	2,607
Income tax expense (benefit)									398	403	528
Net income from continuing operations									2,096	1,398	2,079
Net income from discontinued operations									74	828	177
Net income (loss) before equity in earnings of subsidiaries									2,170	2,226	2,256
Equity in earnings of subsidiaries									129	227	409
Net income									2,299	2,453	2,665
Less: Net income attributable to noncontrolling interests									40	37	34
Net income attributable to Aon shareholders									2,259	2,416	2,631
Consolidating Adjustments
Revenue
Total revenue									0	0	0
Expenses
Compensation and benefits									0	0	0
Information technology									0	0	0
Premises									0	0	0
Depreciation of fixed assets									0	0	0
Amortization and impairment of intangible assets									0	0	0
Other general expenses									0	0	0
Total operating expenses									0	0	0
Operating income									0	0	0
Interest income									(53)	(29)	(29)
Interest expense									53	29	29
Intercompany interest income (expense)									0	0	0
Intercompany other income (expense)									0	0	0
Other income (expense)									(21)	18	(18)
Income from continuing operations before income taxes									(21)	18	(18)
Income tax expense (benefit)									0	0	0
Net income from continuing operations									(21)	18	(18)
Net income from discontinued operations									0	0	0
Net income (loss) before equity in earnings of subsidiaries									(21)	18	(18)
Equity in earnings of subsidiaries									(2,388)	(2,643)	(3,040)
Net income									(2,409)	(2,625)	(3,058)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									$ (2,409)	$ (2,625)	$ (3,058)