Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Condensed Financial Statements, Captions [Line Items]
Net income	$ 303	$ 315	$ 832	$ 964
Less: Net income attributable to noncontrolling interests	8	6	31	26
Net income attributable to Aon shareholders	295	309	801	938
Change in fair value of financial instruments	(10)	(8)	(11)	12
Foreign currency translation adjustments	(229)	(318)	(376)	(228)
Post-retirement benefit obligation	18	26	62	70
Total other comprehensive (loss)	(221)	(300)	(325)	(146)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive (loss) income attributable to noncontrolling interests	(4)	0	(6)	(2)
Total other comprehensive (loss) attributable to Aon shareholders	(217)	(300)	(319)	(144)
Comprehensive income attributable to Aon shareholders	78	9	482	794
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	295	309	801	938
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	295	309	801	938
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	0	0	0	0
Post-retirement benefit obligation	0	0	0	0
Total other comprehensive (loss)	0	0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	(217)	(300)	(319)	(144)
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) attributable to Aon shareholders	(217)	(300)	(319)	(144)
Comprehensive income attributable to Aon shareholders	78	9	482	794
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	234	162	600	635
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	234	162	600	635
Change in fair value of financial instruments	(1)	(1)	0	(1)
Foreign currency translation adjustments	(24)	(17)	(43)	(19)
Post-retirement benefit obligation	8	6	25	17
Total other comprehensive (loss)	(17)	(12)	(18)	(3)
Equity in other comprehensive loss of subsidiaries, net of tax	(196)	(285)	(290)	(142)
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) attributable to Aon shareholders	(213)	(297)	(308)	(145)
Comprehensive income attributable to Aon shareholders	21	(135)	292	490
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	662	533	1,757	1,767
Less: Net income attributable to noncontrolling interests	8	6	31	26
Net income attributable to Aon shareholders	654	527	1,726	1,741
Change in fair value of financial instruments	(9)	(7)	(11)	13
Foreign currency translation adjustments	(205)	(301)	(333)	(209)
Post-retirement benefit obligation	10	20	37	53
Total other comprehensive (loss)	(204)	(288)	(307)	(143)
Equity in other comprehensive loss of subsidiaries, net of tax	(213)	0	(308)	0
Less: Other comprehensive (loss) income attributable to noncontrolling interests	(4)	0	(6)	(2)
Total other comprehensive (loss) attributable to Aon shareholders	(413)	(288)	(609)	(141)
Comprehensive income attributable to Aon shareholders	241	239	1,117	1,600
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(888)	(689)	(2,326)	(2,376)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(888)	(689)	(2,326)	(2,376)
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	0	0	0	0
Post-retirement benefit obligation	0	0	0	0
Total other comprehensive (loss)	0	0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	626	585	917	286
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) attributable to Aon shareholders	626	585	917	286
Comprehensive income attributable to Aon shareholders	$ (262)	$ (104)	$ (1,409)	$ (2,090)