Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue
Total revenue	$ 2,349	$ 2,340	$ 8,000	$ 7,089
Expenses
Compensation and benefits	1,392	1,428	4,502	4,363
Information technology	125	109	363	295
Premises	94	89	283	259
Depreciation of fixed assets	40	40	126	148
Amortization and impairment of intangible assets	100	101	492	604
Other general expenses	336	317	1,189	956
Total operating expenses	2,087	2,084	6,955	6,625
Operating income	262	256	1,045	464
Interest income	0	10	5	20
Interest expense	(69)	(70)	(208)	(211)
Intercompany interest income (expense)	0	0	0	0
Intercompany other income (expense)	0	0	0	0
Other income (expense)	1	4	(17)	6
Income from continuing operations before income taxes	194	200	825	279
Income tax expense (benefit)	39	4	9	(139)
Net income from continuing operations	155	196	816	418
Net income (loss) from discontinued operations	(2)	(4)	5	857
Net income (loss) before equity in earnings of subsidiaries	153	192	821	1,275
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	153	192	821	1,275
Less: Net income attributable to noncontrolling interests	6	7	32	30
Net income attributable to Aon shareholders	147	185	789	1,245
Aon plc
Revenue
Total revenue	0	0	0	0
Expenses
Compensation and benefits	13	25	47	85
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	1	3	10
Total operating expenses	13	26	50	95
Operating income	(13)	(26)	(50)	(95)
Interest income	0	0	0	0
Interest expense	(52)	(53)	(149)	(144)
Intercompany interest income (expense)	4	3	11	10
Intercompany other income (expense)	245	291	113	189
Other income (expense)	(5)	(2)	4	(25)
Income from continuing operations before income taxes	179	213	(71)	(65)
Income tax expense (benefit)	(8)	(8)	(27)	(30)
Net income from continuing operations	187	221	(44)	(35)
Net income (loss) from discontinued operations	0	0	0	0
Net income (loss) before equity in earnings of subsidiaries	187	221	(44)	(35)
Equity in earnings of subsidiaries, net of tax	(42)	(36)	846	1,262
Net income	145	185	802	1,227
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	145	185	802	1,227
Aon Corporation
Revenue
Total revenue	0	0	0	0
Expenses
Compensation and benefits	3	18	5	24
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	(7)	1	71	(3)
Total operating expenses	(4)	19	76	21
Operating income	4	(19)	(76)	(21)
Interest income	15	18	44	35
Interest expense	(26)	(24)	(75)	(71)
Intercompany interest income (expense)	(128)	(135)	(385)	(407)
Intercompany other income (expense)	(251)	(271)	(259)	(280)
Other income (expense)	(3)	12	(29)	15
Income from continuing operations before income taxes	(389)	(419)	(780)	(729)
Income tax expense (benefit)	(67)	(81)	(144)	(198)
Net income from continuing operations	(322)	(338)	(636)	(531)
Net income (loss) from discontinued operations	0	0	0	0
Net income (loss) before equity in earnings of subsidiaries	(322)	(338)	(636)	(531)
Equity in earnings of subsidiaries, net of tax	(42)	122	868	1,028
Net income	(364)	(216)	232	497
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(364)	(216)	232	497
Other Non-Guarantor Subsidiaries
Revenue
Total revenue	2,349	2,340	8,000	7,089
Expenses
Compensation and benefits	1,376	1,385	4,450	4,254
Information technology	125	109	363	295
Premises	94	89	283	259
Depreciation of fixed assets	40	40	126	148
Amortization and impairment of intangible assets	100	101	492	604
Other general expenses	343	315	1,115	949
Total operating expenses	2,078	2,039	6,829	6,509
Operating income	271	301	1,171	580
Interest income	0	0	0	0
Interest expense	(6)	(1)	(23)	(11)
Intercompany interest income (expense)	124	132	374	397
Intercompany other income (expense)	6	(20)	146	91
Other income (expense)	7	(6)	21	(2)
Income from continuing operations before income taxes	402	406	1,689	1,055
Income tax expense (benefit)	114	93	180	89
Net income from continuing operations	288	313	1,509	966
Net income (loss) from discontinued operations	(2)	(4)	5	857
Net income (loss) before equity in earnings of subsidiaries	286	309	1,514	1,823
Equity in earnings of subsidiaries, net of tax	(364)	(216)	232	497
Net income	(78)	93	1,746	2,320
Less: Net income attributable to noncontrolling interests	6	7	32	30
Net income attributable to Aon shareholders	(84)	86	1,714	2,290
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	(15)	(8)	(39)	(15)
Interest expense	15	8	39	15
Intercompany interest income (expense)	0	0	0	0
Intercompany other income (expense)	0	0	0	0
Other income (expense)	2	0	(13)	18
Income from continuing operations before income taxes	2	0	(13)	18
Income tax expense (benefit)	0	0	0	0
Net income from continuing operations	2	0	(13)	18
Net income (loss) from discontinued operations	0	0	0	0
Net income (loss) before equity in earnings of subsidiaries	2	0	(13)	18
Equity in earnings of subsidiaries, net of tax	448	130	(1,946)	(2,787)
Net income	450	130	(1,959)	(2,769)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ 450	$ 130	$ (1,959)	$ (2,769)