Guarantee of Registered Securities Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Condensed Financial Statements, Captions [Line Items]
Net income	$ 264	$ 210	$ 788	$ 713
Less: Net income attributable to noncontrolling interests	8	6	30	25
Net income attributable to Aon shareholders	256	204	758	688
Change in fair value of investments	(14)	0	(1)	0
Change in fair value of derivatives	9	15	(12)	5
Foreign currency translation adjustments	155	164	(77)	71
Post-retirement benefit obligation	24	7	65	46
Total other comprehensive income (loss)	174	186	(25)	122
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive income attributable to noncontrolling interests	3	4	1	2
Total other comprehensive income (loss) attributable to Aon shareholders	171	182	(26)	120
Comprehensive income attributable to Aon shareholders	427	386	732	808
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	256	204	758	688
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	256	204	758	688
Change in fair value of investments	0	0	0	0
Change in fair value of derivatives	0	0	0	0
Foreign currency translation adjustments	0	0	0	0
Post-retirement benefit obligation	0	0	0	0
Total other comprehensive income (loss)	0	0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	170	182	(27)	120
Less: Other comprehensive income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	170	182	(27)	120
Comprehensive income attributable to Aon shareholders	426	386	731	808
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	108	182	580	582
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	108	182	580	582
Change in fair value of investments	(4)	0	0	0
Change in fair value of derivatives	2	1	5	1
Foreign currency translation adjustments	6	17	(14)	16
Post-retirement benefit obligation	7	9	22	24
Total other comprehensive income (loss)	11	27	13	41
Equity in other comprehensive loss of subsidiaries, net of tax	157	168	(38)	92
Less: Other comprehensive income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	168	195	(25)	133
Comprehensive income attributable to Aon shareholders	276	377	555	715
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	272	242	853	756
Less: Net income attributable to noncontrolling interests	8	6	30	25
Net income attributable to Aon shareholders	264	236	823	731
Change in fair value of investments	(10)	0	(1)	0
Change in fair value of derivatives	7	14	(17)	4
Foreign currency translation adjustments	149	147	(63)	55
Post-retirement benefit obligation	17	(2)	43	22
Total other comprehensive income (loss)	163	159	(38)	81
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive income attributable to noncontrolling interests	3	4	1	2
Total other comprehensive income (loss) attributable to Aon shareholders	160	155	(39)	79
Comprehensive income attributable to Aon shareholders	424	391	784	810
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(372)	(418)	(1,403)	(1,313)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(372)	(418)	(1,403)	(1,313)
Change in fair value of investments	0	0	0	0
Change in fair value of derivatives	0	0	0	0
Foreign currency translation adjustments	0	0	0	0
Post-retirement benefit obligation	0	0	0	0
Total other comprehensive income (loss)	0	0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	(327)	(350)	65	(212)
Less: Other comprehensive income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(327)	(350)	65	(212)
Comprehensive income attributable to Aon shareholders	$ (699)	$ (768)	$ (1,338)	$ (1,525)