Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ 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
Revenue
Total revenue	$ 2,909	$ 2,340	$ 2,368	$ 2,381	$ 2,650	$ 2,201	$ 2,282	$ 2,276	$ 9,998	$ 9,409	$ 9,480
Expenses
Compensation and benefits									6,089	5,687	5,605
Information technology									419	386	389
Premises									348	343	362
Depreciation of fixed assets									187	162	164
Amortization and impairment of intangible assets									704	157	173
Other general expenses									1,272	1,036	1,200
Total operating expenses									9,019	7,771	7,893
Operating income	489	265	(118)	343	463	368	387	420	979	1,638	1,587
Interest income									27	9	14
Interest expense									(282)	(282)	(273)
Intercompany interest income (expense)									0	0	0
Intercompany other income (expense)									0	0	0
Other income (expense)									(39)	36	100
Income from continuing operations before income taxes									685	1,401	1,428
Income tax expense (benefit)									250	148	175
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
Income (loss) before equity earnings of subsidiaries									1,263	1,430	1,422
Equity in earnings of subsidiaries, net of tax									0	0	0
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
Aon plc
Revenue
Total revenue									0	0	0
Expenses
Compensation and benefits									150	130	136
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									12	0	8
Total operating expenses									162	130	144
Operating income									(162)	(130)	(144)
Interest income									0	0	0
Interest expense									(202)	(196)	(140)
Intercompany interest income (expense)									14	14	429
Intercompany other income (expense)									247	274	302
Other income (expense)									(27)	15	(1)
Income from continuing operations before income taxes									(130)	(23)	446
Income tax expense (benefit)									(43)	(55)	45
Net income from continuing operations									(87)	32	401
Income from discontinued operations, net of tax									0	0	0
Income (loss) before equity earnings of subsidiaries									(87)	32	401
Equity in earnings of subsidiaries, net of tax									1,295	1,382	984
Net income									1,208	1,414	1,385
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									1,208	1,414	1,385
Aon Corporation
Revenue
Total revenue									0	0	0
Expenses
Compensation and benefits									35	171	32
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									(6)	2	7
Total operating expenses									29	173	39
Operating income									(29)	(173)	(39)
Interest income									52	16	14
Interest expense									(94)	(101)	(130)
Intercompany interest income (expense)									(543)	(541)	(479)
Intercompany other income (expense)									(411)	(361)	(422)
Other income (expense)									21	(5)	0
Income from continuing operations before income taxes									(1,004)	(1,165)	(1,056)
Income tax expense (benefit)									(110)	(325)	(262)
Net income from continuing operations									(894)	(840)	(794)
Income from discontinued operations, net of tax									0	0	0
Income (loss) before equity earnings of subsidiaries									(894)	(840)	(794)
Equity in earnings of subsidiaries, net of tax									1,054	1,219	1,289
Net income									160	379	495
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									160	379	495
Other Non-Guarantor Subsidiaries
Revenue
Total revenue									9,998	9,409	9,480
Expenses
Compensation and benefits									5,904	5,386	5,437
Information technology									419	386	389
Premises									348	343	362
Depreciation of fixed assets									187	162	164
Amortization and impairment of intangible assets									704	157	173
Other general expenses									1,266	1,034	1,185
Total operating expenses									8,828	7,468	7,710
Operating income									1,170	1,941	1,770
Interest income									4	22	19
Interest expense									(15)	(14)	(22)
Intercompany interest income (expense)									529	527	50
Intercompany other income (expense)									164	87	120
Other income (expense)									(51)	44	101
Income from continuing operations before income taxes									1,801	2,607	2,038
Income tax expense (benefit)									403	528	392
Net income from continuing operations									1,398	2,079	1,646
Income from discontinued operations, net of tax									828	177	169
Income (loss) before equity earnings of subsidiaries									2,226	2,256	1,815
Equity in earnings of subsidiaries, net of tax									160	379	495
Net income									2,386	2,635	2,310
Less: Net income attributable to noncontrolling interests									37	34	37
Net income attributable to Aon shareholders									2,349	2,601	2,273
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									(29)	(29)	(19)
Interest expense									29	29	19
Intercompany interest income (expense)									0	0	0
Intercompany other income (expense)									0	0	0
Other income (expense)									18	(18)	0
Income from continuing operations before income taxes									18	(18)	0
Income tax expense (benefit)									0	0	0
Net income from continuing operations									18	(18)	0
Income from discontinued operations, net of tax									0	0	0
Income (loss) before equity earnings of subsidiaries									18	(18)	0
Equity in earnings of subsidiaries, net of tax									(2,509)	(2,980)	(2,768)
Net income									(2,491)	(2,998)	(2,768)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									$ (2,491)	$ (2,998)	$ (2,768)