Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended								6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Deferred tax assets	$ 322				$ 300						$ 322	$ 300
Revenue
Commissions, fees and other	3,256	$ 2,754	$ 2,795	$ 2,800	3,222	$ 2,746	$ 2,831	$ 2,862			11,605	11,661	$ 12,019
Fiduciary investment income	6	6	5	5	5	6	5	5			22	21	26
Total revenue	3,262	2,760	2,800	2,805	3,227	2,752	2,836	2,867			11,627	11,682	12,045
Expenses
Compensation and benefits											6,914	6,837	7,014
Other general expenses											2,807	2,997	3,065
Total operating expenses											9,721	9,834	10,079
Operating income	568	436	439	463	656	423	308	461			1,906	1,848	1,966
Interest income											9	14	10
Interest expense											(282)	(273)	(255)
Intercompany interest income (expense)											0	0	0
Intercompany other income (expense)											0	0	0
Other income											36	100	44
Income before income taxes											1,669	1,689	1,765
Income tax expense (benefit)											239	267	334
Income (loss) before equity in earnings of subsidiaries											1,430	1,422	1,431
Equity in earnings of subsidiaries, net of tax											0	0	0
Net income	459	326	308	337	540	311	214	357	$ 645	$ 971	1,430	1,422	1,431
Less: Net income attributable to noncontrolling interests	7	7	8	12	6	8	10	13			34	37	34
Net income attributable to Aon shareholders	452	$ 319	$ 300	$ 325	534	$ 303	$ 204	$ 344			1,396	1,385	1,397
Aon plc
Condensed Financial Statements, Captions [Line Items]
Deferred tax assets	134				154						134	154
Revenue
Commissions, fees and other											0	0	0
Fiduciary investment income											0	0	0
Total revenue											0	0	0
Expenses
Compensation and benefits											130	136	140
Other general expenses											0	8	3
Total operating expenses											130	144	143
Operating income											(130)	(144)	(143)
Interest income											0	0	0
Interest expense											(196)	(140)	(84)
Intercompany interest income (expense)											14	429	449
Intercompany other income (expense)											274	302	342
Other income											15	(1)	2
Income before income taxes											(23)	446	566
Income tax expense (benefit)											(55)	45	74
Income (loss) before equity in earnings of subsidiaries											32	401	492
Equity in earnings of subsidiaries, net of tax											1,382	984	905
Net income											1,414	1,385	1,397
Less: Net income attributable to noncontrolling interests											0	0	0
Net income attributable to Aon shareholders											1,414	1,385	1,397
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Deferred tax assets	726				756						726	756
Revenue
Commissions, fees and other											0	0	0
Fiduciary investment income											0	0	0
Total revenue											0	0	0
Expenses
Compensation and benefits											171	32	16
Other general expenses											2	7	5
Total operating expenses											173	39	21
Operating income											(173)	(39)	(21)
Interest income											16	14	2
Interest expense											(101)	(130)	(139)
Intercompany interest income (expense)											(541)	(479)	(298)
Intercompany other income (expense)											(361)	(422)	(390)
Other income											(5)	0	5
Income before income taxes											(1,165)	(1,056)	(841)
Income tax expense (benefit)											(325)	(262)	(192)
Income (loss) before equity in earnings of subsidiaries											(840)	(794)	(649)
Equity in earnings of subsidiaries, net of tax											1,219	1,290	1,195
Net income											379	496	546
Less: Net income attributable to noncontrolling interests											0	0	0
Net income attributable to Aon shareholders											379	496	546
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Deferred tax assets	168				207						168	207
Revenue
Commissions, fees and other											11,605	11,661	12,019
Fiduciary investment income											22	21	26
Total revenue											11,627	11,682	12,045
Expenses
Compensation and benefits											6,613	6,669	6,858
Other general expenses											2,805	2,982	3,057
Total operating expenses											9,418	9,651	9,915
Operating income											2,209	2,031	2,130
Interest income											22	19	17
Interest expense											(14)	(22)	(41)
Intercompany interest income (expense)											527	50	(151)
Intercompany other income (expense)											87	120	48
Other income											44	101	37
Income before income taxes											2,875	2,299	2,040
Income tax expense (benefit)											619	484	452
Income (loss) before equity in earnings of subsidiaries											2,256	1,815	1,588
Equity in earnings of subsidiaries, net of tax											379	496	546
Net income											2,635	2,311	2,134
Less: Net income attributable to noncontrolling interests											34	37	34
Net income attributable to Aon shareholders											2,601	2,274	2,100
Consolidating Adjustments
Condensed Financial Statements, Captions [Line Items]
Deferred tax assets	$ (706)				$ (817)						(706)	(817)
Revenue
Commissions, fees and other											0	0	0
Fiduciary investment income											0	0	0
Total revenue											0	0	0
Expenses
Compensation and benefits											0	0	0
Other general expenses											0	0	0
Total operating expenses											0	0	0
Operating income											0	0	0
Interest income											(29)	(19)	(9)
Interest expense											29	19	9
Intercompany interest income (expense)											0	0	0
Intercompany other income (expense)											0	0	0
Other income											(18)	0	0
Income before income taxes											(18)	0	0
Income tax expense (benefit)											0	0	0
Income (loss) before equity in earnings of subsidiaries											(18)	0	0
Equity in earnings of subsidiaries, net of tax											(2,980)	(2,770)	(2,646)
Net income											(2,998)	(2,770)	(2,646)
Less: Net income attributable to noncontrolling interests											0	0	0
Net income attributable to Aon shareholders											$ (2,998)	$ (2,770)	$ (2,646)