Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net income	$ 467	$ 315	$ 313	$ 336	$ 360	$ 264	$ 252	$ 272	$ 1,431	$ 1,148	$ 1,020
Less: Net income attributable to noncontrolling interests	8	6	9	11	5	8	11	11	34	35	27
Net income attributable to Aon shareholders	459	309	304	325	355	256	241	261	1,397	1,113	993
Change in fair value of investments									(1)	1	0
Change in fair value of derivatives									5	6	9
Foreign currency translation adjustments									(507)	(65)	109
Post-retirement benefit obligation									(260)	293	(358)
Total other comprehensive loss									(763)	235	(240)
Equity in other comprehensive loss of subsidiaries, net of tax									0	0	0
Less: Other comprehensive (loss) income attributable to noncontrolling interests									(3)	(1)	0
Total other comprehensive (loss) income attributable to Aon shareholders									(760)	236	(240)
Comprehensive income attributable to Aon shareholders									637	1,349	753
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									1,397	1,113	993
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									1,397	1,113	993
Change in fair value of investments									0	0
Change in fair value of derivatives									0	0	0
Foreign currency translation adjustments									0	0	0
Post-retirement benefit obligation									0	0	0
Total other comprehensive loss									0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax									(760)	236	(240)
Less: Other comprehensive (loss) income attributable to noncontrolling interests									0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders									(760)	236	(240)
Comprehensive income attributable to Aon shareholders									637	1,349	753
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									565	815	939
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									565	815	939
Change in fair value of investments									0	0
Change in fair value of derivatives									(3)	5	2
Foreign currency translation adjustments									(31)	(60)	24
Post-retirement benefit obligation									(315)	223	(68)
Total other comprehensive loss									(349)	168	(42)
Equity in other comprehensive loss of subsidiaries, net of tax									(411)	69	(187)
Less: Other comprehensive (loss) income attributable to noncontrolling interests									0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders									(760)	237	(229)
Comprehensive income attributable to Aon shareholders									(195)	1,052	710
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									1,588	1,376	1,128
Less: Net income attributable to noncontrolling interests									34	35	27
Net income attributable to Aon shareholders									1,554	1,341	1,101
Change in fair value of investments									(1)	1
Change in fair value of derivatives									8	1	7
Foreign currency translation adjustments									(476)	(5)	85
Post-retirement benefit obligation									55	70	(290)
Total other comprehensive loss									(414)	67	(198)
Equity in other comprehensive loss of subsidiaries, net of tax									0	0	0
Less: Other comprehensive (loss) income attributable to noncontrolling interests									(3)	(1)	0
Total other comprehensive (loss) income attributable to Aon shareholders									(411)	68	(198)
Comprehensive income attributable to Aon shareholders									1,143	1,409	903
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									(2,119)	(2,156)	(2,040)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									(2,119)	(2,156)	(2,040)
Change in fair value of investments									0	0
Change in fair value of derivatives									0	0	0
Foreign currency translation adjustments									0	0	0
Post-retirement benefit obligation									0	0	0
Total other comprehensive loss									0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax									1,171	(305)	427
Less: Other comprehensive (loss) income attributable to noncontrolling interests									0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders									1,171	(305)	427
Comprehensive income attributable to Aon shareholders									$ (948)	$ (2,461)	$ (1,613)