Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue
Total revenue	$ 2,561	$ 2,368	$ 5,651	$ 4,749
Expenses
Compensation and benefits	1,494	1,466	3,110	2,935
Information technology	123	98	238	186
Premises	96	86	189	170
Depreciation of fixed assets	47	54	86	108
Amortization and impairment of intangible assets	282	460	392	503
Other general expenses	535	331	853	639
Total operating expenses	2,577	2,495	4,868	4,541
Operating income (loss)	(16)	(127)	783	208
Interest income	1	8	5	10
Interest expense	(69)	(71)	(139)	(141)
Intercompany interest income (expense)	0	0	0	0
Intercompany other income (expense)	0	0	0	0
Other income (expense)	(3)	4	(18)	2
Income (loss) from continuing operations before income taxes	(87)	(186)	631	79
Income tax expense (benefit)	(144)	(143)	(30)	(143)
Net income (loss) from continuing operations	57	(43)	661	222
Net income from discontinued operations	1	821	7	861
Net income (loss) before equity in earnings of subsidiaries	58	778	668	1,083
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	58	778	668	1,083
Less: Net income attributable to noncontrolling interests	10	9	26	23
Net income attributable to Aon shareholders	48	769	642	1,060
Aon plc
Revenue
Total revenue	0	0	0	0
Expenses
Compensation and benefits	15	8	34	60
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	2	4	3	9
Total operating expenses	17	12	37	69
Operating income (loss)	(17)	(12)	(37)	(69)
Interest income	0	0	0	0
Interest expense	(48)	(46)	(97)	(91)
Intercompany interest income (expense)	3	4	7	7
Intercompany other income (expense)	(61)	(53)	(132)	(102)
Other income (expense)	16	(12)	9	(23)
Income (loss) from continuing operations before income taxes	(107)	(119)	(250)	(278)
Income tax expense (benefit)	(3)	(8)	(19)	(22)
Net income (loss) from continuing operations	(104)	(111)	(231)	(256)
Net income from discontinued operations	0	0	0	0
Net income (loss) before equity in earnings of subsidiaries	(104)	(111)	(231)	(256)
Equity in earnings of subsidiaries, net of tax	170	870	888	1,298
Net income	66	759	657	1,042
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	66	759	657	1,042
Aon Corporation
Revenue
Total revenue	0	0	0	0
Expenses
Compensation and benefits	1	3	2	6
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	78	(6)	78	(4)
Total operating expenses	79	(3)	80	2
Operating income (loss)	(79)	3	(80)	(2)
Interest income	15	11	29	17
Interest expense	(25)	(23)	(49)	(47)
Intercompany interest income (expense)	(129)	(136)	(257)	(272)
Intercompany other income (expense)	(3)	(16)	(8)	(9)
Other income (expense)	(20)	(6)	(26)	3
Income (loss) from continuing operations before income taxes	(241)	(167)	(391)	(310)
Income tax expense (benefit)	(50)	(63)	(77)	(117)
Net income (loss) from continuing operations	(191)	(104)	(314)	(193)
Net income from discontinued operations	0	0	0	0
Net income (loss) before equity in earnings of subsidiaries	(191)	(104)	(314)	(193)
Equity in earnings of subsidiaries, net of tax	206	635	910	906
Net income	15	531	596	713
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	15	531	596	713
Other Non-Guarantor Subsidiaries
Revenue
Total revenue	2,561	2,368	5,651	4,749
Expenses
Compensation and benefits	1,478	1,455	3,074	2,869
Information technology	123	98	238	186
Premises	96	86	189	170
Depreciation of fixed assets	47	54	86	108
Amortization and impairment of intangible assets	282	460	392	503
Other general expenses	455	333	772	634
Total operating expenses	2,481	2,486	4,751	4,470
Operating income (loss)	80	(118)	900	279
Interest income	0	2	0	0
Interest expense	(10)	(7)	(17)	(10)
Intercompany interest income (expense)	126	132	250	265
Intercompany other income (expense)	64	69	140	111
Other income (expense)	19	12	14	4
Income (loss) from continuing operations before income taxes	279	90	1,287	649
Income tax expense (benefit)	(91)	(72)	66	(4)
Net income (loss) from continuing operations	370	162	1,221	653
Net income from discontinued operations	1	821	7	861
Net income (loss) before equity in earnings of subsidiaries	371	983	1,228	1,514
Equity in earnings of subsidiaries, net of tax	15	531	596	713
Net income	386	1,514	1,824	2,227
Less: Net income attributable to noncontrolling interests	10	9	26	23
Net income attributable to Aon shareholders	376	1,505	1,798	2,204
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 (loss)	0	0	0	0
Interest income	(14)	(5)	(24)	(7)
Interest expense	14	5	24	7
Intercompany interest income (expense)	0	0	0	0
Intercompany other income (expense)	0	0	0	0
Other income (expense)	(18)	10	(15)	18
Income (loss) from continuing operations before income taxes	(18)	10	(15)	18
Income tax expense (benefit)	0	0	0	0
Net income (loss) from continuing operations	(18)	10	(15)	18
Net income from discontinued operations	0	0	0	0
Net income (loss) before equity in earnings of subsidiaries	(18)	10	(15)	18
Equity in earnings of subsidiaries, net of tax	(391)	(2,036)	(2,394)	(2,917)
Net income	(409)	(2,026)	(2,409)	(2,899)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ (409)	$ (2,026)	$ (2,409)	$ (2,899)