Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue
Total revenue	$ 2,340	$ 2,201	$ 7,089	$ 6,759
Expenses
Compensation and benefits	1,419	1,300	4,337	4,041
Information technology	109	99	295	281
Premises	89	86	259	257
Depreciation of fixed assets	40	39	148	118
Amortization and impairment of intangible assets	101	42	604	117
Other general expenses	317	267	956	770
Total operating expenses	2,075	1,833	6,599	5,584
Operating income	265	368	490	1,175
Interest income	10	1	20	6
Interest expense	(70)	(70)	(211)	(212)
Intercompany interest income (expense)	0	0	0	0
Intercompany other income (expense)	0	0	0	0
Other income (expense)	(5)	10	(20)	27
Income from continuing operations before income taxes	200	309	279	996
Income tax benefit (expense)	4	25	(139)	127
Net income from continuing operations	196	284	418	869
Income (loss) from discontinued operations, net of tax	(4)	42	857	102
Net income (loss) before equity in earnings of subsidiaries	192	326	1,275	971
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	192	326	1,275	971
Less: Net income attributable to noncontrolling interests	7	7	30	27
Net income attributable to Aon shareholders	185	319	1,245	944
Aon plc
Revenue
Total revenue	0	0	0	0
Expenses
Compensation and benefits	25	25	85	76
Information technology	0	0	0	0
Premises	0	0	0	0
Depreciation of fixed assets	0	0	0	0
Amortization and impairment of intangible assets	0	0	0	0
Other general expenses	1	(1)	10	5
Total operating expenses	26	24	95	81
Operating income	(26)	(24)	(95)	(81)
Interest income	0	0	0	0
Interest expense	(53)	(51)	(144)	(145)
Intercompany interest income (expense)	3	3	10	10
Intercompany other income (expense)	291	328	189	217
Other income (expense)	(2)	(5)	(25)	(3)
Income from continuing operations before income taxes	213	251	(65)	(2)
Income tax benefit (expense)	(8)	13	(30)	(33)
Net income from continuing operations	221	238	(35)	31
Income (loss) from discontinued operations, net of tax	0	0	0	0
Net income (loss) before equity in earnings of subsidiaries	221	238	(35)	31
Equity in earnings of subsidiaries, net of tax	(36)	78	1,262	914
Net income	185	316	1,227	945
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	185	316	1,227	945
Aon Corporation
Revenue
Total revenue	0	0	0	0
Expenses
Compensation and benefits	20	4	31	10
Information technology	0	0	0	0
Premises	0	0	0	0
Depreciation of fixed assets	0	0	0	0
Amortization and impairment of intangible assets	0	0	0	0
Other general expenses	1	3	(3)	7
Total operating expenses	21	7	28	17
Operating income	(21)	(7)	(28)	(17)
Interest income	18	4	35	13
Interest expense	(24)	(24)	(71)	(78)
Intercompany interest income (expense)	(135)	(135)	(407)	(405)
Intercompany other income (expense)	(271)	(277)	(280)	(292)
Other income (expense)	14	1	22	(8)
Income from continuing operations before income taxes	(419)	(438)	(729)	(787)
Income tax benefit (expense)	(81)	(93)	(198)	(219)
Net income from continuing operations	(338)	(345)	(531)	(568)
Income (loss) from discontinued operations, net of tax	0	0	0	0
Net income (loss) before equity in earnings of subsidiaries	(338)	(345)	(531)	(568)
Equity in earnings of subsidiaries, net of tax	122	225	1,028	836
Net income	(216)	(120)	497	268
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(216)	(120)	497	268
Other Non-Guarantor Subsidiaries
Revenue
Total revenue	2,340	2,201	7,089	6,759
Expenses
Compensation and benefits	1,374	1,271	4,221	3,955
Information technology	109	99	295	281
Premises	89	86	259	257
Depreciation of fixed assets	40	39	148	118
Amortization and impairment of intangible assets	101	42	604	117
Other general expenses	315	265	949	758
Total operating expenses	2,028	1,802	6,476	5,486
Operating income	312	399	613	1,273
Interest income	0	5	0	14
Interest expense	(1)	(3)	(11)	(10)
Intercompany interest income (expense)	132	132	397	395
Intercompany other income (expense)	(20)	(51)	91	75
Other income (expense)	(17)	11	(35)	39
Income from continuing operations before income taxes	406	493	1,055	1,786
Income tax benefit (expense)	93	105	89	379
Net income from continuing operations	313	388	966	1,407
Income (loss) from discontinued operations, net of tax	(4)	42	857	102
Net income (loss) before equity in earnings of subsidiaries	309	430	1,823	1,509
Equity in earnings of subsidiaries, net of tax	(216)	(120)	497	268
Net income	93	310	2,320	1,777
Less: Net income attributable to noncontrolling interests	7	7	30	27
Net income attributable to Aon shareholders	86	303	2,290	1,750
Consolidation Adjustments
Revenue
Total revenue	0	0	0	0
Expenses
Compensation and benefits	0	0	0	0
Information technology	0	0	0	0
Premises	0	0	0	0
Depreciation of fixed assets	0	0	0	0
Amortization and impairment of intangible assets	0	0	0	0
Other general expenses	0	0	0	0
Total operating expenses	0	0	0	0
Operating income	0	0	0	0
Interest income	(8)	(8)	(15)	(21)
Interest expense	8	8	15	21
Intercompany interest income (expense)	0	0	0	0
Intercompany other income (expense)	0	0	0	0
Other income (expense)	0	3	18	(1)
Income from continuing operations before income taxes	0	3	18	(1)
Income tax benefit (expense)	0	0	0	0
Net income from continuing operations	0	3	18	(1)
Income (loss) from discontinued operations, net of tax	0	0	0	0
Net income (loss) before equity in earnings of subsidiaries	0	3	18	(1)
Equity in earnings of subsidiaries, net of tax	130	(183)	(2,787)	(2,018)
Net income	130	(180)	(2,769)	(2,019)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ 130	$ (180)	$ (2,769)	$ (2,019)