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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]
Net income	$ 360	$ 264	$ 252	$ 272	$ 307	$ 210	$ 254	$ 249	$ 1,148	$ 1,020	$ 1,010
Less: Net income attributable to noncontrolling interests	5	8	11	11	2	6	8	11	35	27	31
Net income attributable to Aon shareholders	355	256	241	261	305	204	246	238	1,113	993	979
Change in fair value of investments									1	0	0
Change in fair value of derivatives									6	9	(13)
Foreign currency translation adjustments									(65)	109	(43)
Post-retirement benefit obligation									293	(358)	(396)
Total other comprehensive gain (loss)									235	(240)	(452)
Equity in other comprehensive income of subsidiaries, net of tax									0	0	0
Less: Other comprehensive (loss) income attributable to noncontrolling interests									(1)	0	1
Total other comprehensive gain (loss) attributable to Aon shareholders									236	(240)	(453)
Comprehensive income attributable to Aon shareholders									1,349	753	526
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									1,113	993	979
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									1,113	993	979
Change in fair value of investments									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 gain (loss)									0	0	0
Equity in other comprehensive income of subsidiaries, net of tax									236	(240)	(453)
Less: Other comprehensive (loss) income attributable to noncontrolling interests									0	0	0
Total other comprehensive gain (loss) attributable to Aon shareholders									236	(240)	(453)
Comprehensive income attributable to Aon shareholders									1,349	753	526
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									815	939	797
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									815	939	797
Change in fair value of investments									0
Change in fair value of derivatives									5	2	0
Foreign currency translation adjustments									(60)	24	(6)
Post-retirement benefit obligation									223	(68)	(173)
Total other comprehensive gain (loss)									168	(42)	(179)
Equity in other comprehensive income of subsidiaries, net of tax									69	(187)	(277)
Less: Other comprehensive (loss) income attributable to noncontrolling interests									0	0	0
Total other comprehensive gain (loss) attributable to Aon shareholders									237	(229)	(456)
Comprehensive income attributable to Aon shareholders									1,052	710	341
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									1,376	1,128	1,081
Less: Net income attributable to noncontrolling interests									35	27	31
Net income attributable to Aon shareholders									1,341	1,101	1,050
Change in fair value of investments									1
Change in fair value of derivatives									1	7	(13)
Foreign currency translation adjustments									(5)	85	(37)
Post-retirement benefit obligation									70	(290)	(223)
Total other comprehensive gain (loss)									67	(198)	(273)
Equity in other comprehensive income of subsidiaries, net of tax									0	0	0
Less: Other comprehensive (loss) income attributable to noncontrolling interests									(1)	0	1
Total other comprehensive gain (loss) attributable to Aon shareholders									68	(198)	(274)
Comprehensive income attributable to Aon shareholders									1,409	903	776
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									(2,156)	(2,040)	(1,847)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									(2,156)	(2,040)	(1,847)
Change in fair value of investments									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 gain (loss)									0	0	0
Equity in other comprehensive income of subsidiaries, net of tax									(305)	427	730
Less: Other comprehensive (loss) income attributable to noncontrolling interests									0	0	0
Total other comprehensive gain (loss) attributable to Aon shareholders									(305)	427	730
Comprehensive income attributable to Aon shareholders									$ (2,461)	$ (1,613)	$ (1,117)