Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net income	$ 341	$ 336
Less: Net income attributable to noncontrolling interests	13	11
Net income attributable to Aon shareholders	328	325
Change in fair value of investments	(1)	1
Change in fair value of derivatives	6	9
Foreign currency translation adjustments	(322)	16
Post-retirement benefit obligation	23	26
Total other comprehensive (loss) income	(294)	52
Equity in other comprehensive loss of subsidiaries, net of tax	0	0
Less: Other comprehensive (loss) income attributable to noncontrolling interests	(1)	(2)
Total other comprehensive (loss) income attributable to Aon shareholders	(293)	54
Comprehensive income attributable to Aon shareholders	35	379
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	328	325
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	328	325
Change in fair value of investments	0	0
Change in fair value of derivatives	0	0
Foreign currency translation adjustments	0	0
Post-retirement benefit obligation	0	0
Total other comprehensive (loss) income	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	(293)	54
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	(293)	54
Comprehensive income attributable to Aon shareholders	35	379
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	304	253
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	304	253
Change in fair value of investments	0	0
Change in fair value of derivatives	(1)	1
Foreign currency translation adjustments	(23)	(17)
Post-retirement benefit obligation	8	6
Total other comprehensive (loss) income	(16)	(10)
Equity in other comprehensive loss of subsidiaries, net of tax	(267)	69
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	(283)	59
Comprehensive income attributable to Aon shareholders	21	312
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	763	660
Less: Net income attributable to noncontrolling interests	13	11
Net income attributable to Aon shareholders	750	649
Change in fair value of investments	(1)	1
Change in fair value of derivatives	7	8
Foreign currency translation adjustments	(299)	33
Post-retirement benefit obligation	15	20
Total other comprehensive (loss) income	(278)	62
Equity in other comprehensive loss of subsidiaries, net of tax	(283)	59
Less: Other comprehensive (loss) income attributable to noncontrolling interests	(1)	(2)
Total other comprehensive (loss) income attributable to Aon shareholders	(560)	123
Comprehensive income attributable to Aon shareholders	190	772
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(1,054)	(902)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	(1,054)	(902)
Change in fair value of investments	0	0
Change in fair value of derivatives	0	0
Foreign currency translation adjustments	0	0
Post-retirement benefit obligation	0	0
Total other comprehensive (loss) income	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	843	(182)
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	843	(182)
Comprehensive income attributable to Aon shareholders	$ (211)	$ (1,084)