Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenue
Total revenue	$ 2,368	$ 2,282	$ 4,749	$ 4,558
Expenses
Compensation and benefits	1,457	1,396	2,918	2,741
Information technology	98	99	186	182
Premises	86	89	170	171
Depreciation of fixed assets	54	41	108	79
Amortization and impairment of intangible assets	460	38	503	75
Other general expenses	331	232	639	503
Total operating expenses	2,486	1,895	4,524	3,751
Operating income	(118)	387	225	807
Interest income	8	3	10	5
Interest expense	(71)	(73)	(141)	(142)
Intercompany interest income (expense)	0	0	0	0
Intercompany other income (expense)	0	0	0	0
Other income (expense)	(5)	(1)	(15)	17
Income from continuing operations before income taxes	(186)	316	79	687
Income tax benefit	(143)	43	(143)	102
Net income (loss) from continuing operations	(43)	273	222	585
Income from discontinued operations, net of tax	821	35	861	60
net income (loss) before equity in earnings of subsidiaries	778	308	1,083
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	778	308	1,083	645
Less: Net income attributable to noncontrolling interests	9	8	23	20
Net income attributable to Aon shareholders	769	300	1,060	625
Aon plc
Revenue
Total revenue	0	0	0	0
Expenses
Compensation and benefits	8	8	60	51
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	4	(1)	9	6
Total operating expenses	12	7	69	57
Operating income	(12)	(7)	(69)	(57)
Interest income	0	0	0	0
Interest expense	(46)	(49)	(91)	(94)
Intercompany interest income (expense)	4	3	7	7
Intercompany other income (expense)	(53)	(57)	(102)	(111)
Other income (expense)	(12)	2	(23)	2
Income from continuing operations before income taxes	(119)	(108)	(278)	(253)
Income tax benefit	(8)	(20)	(22)	(46)
Net income (loss) from continuing operations	(111)	(88)	(256)	(207)
Income from discontinued operations, net of tax	0	0	0	0
net income (loss) before equity in earnings of subsidiaries	(111)	(88)	(256)	(207)
Equity in earnings of subsidiaries, net of tax	870	392	1,298	836
Net income	759	304	1,042	629
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	759	304	1,042	629
Aon Corporation
Revenue
Total revenue	0	0	0	0
Expenses
Compensation and benefits	5	3	11	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	(6)	2	(4)	4
Total operating expenses	(1)	5	7	10
Operating income	1	(5)	(7)	(10)
Interest income	11	4	17	9
Interest expense	(23)	(26)	(47)	(54)
Intercompany interest income (expense)	(136)	(137)	(272)	(270)
Intercompany other income (expense)	(16)	(16)	(9)	(15)
Other income (expense)	(4)	(4)	8	(9)
Income from continuing operations before income taxes	(167)	(184)	(310)	(349)
Income tax benefit	(63)	(64)	(117)	(126)
Net income (loss) from continuing operations	(104)	(120)	(193)	(223)
Income from discontinued operations, net of tax	0	0	0	0
net income (loss) before equity in earnings of subsidiaries	(104)	(120)	(193)	(223)
Equity in earnings of subsidiaries, net of tax	635	255	906	611
Net income	531	135	713	388
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	531	135	713	388
Other Non-Guarantor Subsidiaries
Revenue
Total revenue	2,368	2,282	4,749	4,558
Expenses
Compensation and benefits	1,444	1,385	2,847	2,684
Information technology	98	99	186	182
Premises	86	89	170	171
Depreciation of fixed assets	54	41	108	79
Amortization and impairment of intangible assets	460	38	503	75
Other general expenses	333	231	634	493
Total operating expenses	2,475	1,883	4,448	3,684
Operating income	(107)	399	301	874
Interest income	2	5	0	9
Interest expense	(7)	(4)	(10)	(7)
Intercompany interest income (expense)	132	134	265	263
Intercompany other income (expense)	69	73	111	126
Other income (expense)	1	5	(18)	28
Income from continuing operations before income taxes	90	612	649	1,293
Income tax benefit	(72)	127	(4)	274
Net income (loss) from continuing operations	162	485	653	1,019
Income from discontinued operations, net of tax	821	35	861	60
net income (loss) before equity in earnings of subsidiaries	983	520	1,514	1,079
Equity in earnings of subsidiaries, net of tax	531	135	713	388
Net income	1,514	655	2,227	1,467
Less: Net income attributable to noncontrolling interests	9	8	23	20
Net income attributable to Aon shareholders	1,505	647	2,204	1,447
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	(5)	(6)	(7)	(13)
Interest expense	5	6	7	13
Intercompany interest income (expense)	0	0	0	0
Intercompany other income (expense)	0	0	0	0
Other income (expense)	10	(4)	18	(4)
Income from continuing operations before income taxes	10	(4)	18	(4)
Income tax benefit	0	0	0	0
Net income (loss) from continuing operations	10	(4)	18	(4)
Income from discontinued operations, net of tax	0	0	0	0
net income (loss) before equity in earnings of subsidiaries	10	(4)	18	(4)
Equity in earnings of subsidiaries, net of tax	(2,036)	(782)	(2,917)	(1,835)
Net income	(2,026)	(786)	(2,899)	(1,839)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ (2,026)	$ (786)	$ (2,899)	$ (1,839)