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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Condensed Financial Statements, Captions [Line Items]
Net income	$ 315	$ 264	$ 964	$ 788
Less: Net income attributable to noncontrolling interests	6	8	26	30
Net income attributable to Aon shareholders	309	256	938	758
Change in fair value of investments	0	(14)	(1)	(1)
Change in fair value of derivatives	(8)	9	13	(12)
Foreign currency translation adjustments	(318)	155	(228)	(77)
Post-retirement benefit obligation	26	24	70	65
Total other comprehensive (loss) income	(300)	174	(146)	(25)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive income (loss) attributable to noncontrolling interests	0	3	(2)	1
Total other comprehensive (loss) income attributable to Aon shareholders	(300)	171	(144)	(26)
Comprehensive income attributable to Aon shareholders	9	427	794	732
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	309	256	938	758
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	309	256	938	758
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 (loss) income	0	0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	(300)	170	(144)	(27)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	(300)	170	(144)	(27)
Comprehensive income attributable to Aon shareholders	9	426	794	731
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	123	108	508	580
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	123	108	508	580
Change in fair value of investments	0	(4)	0	0
Change in fair value of derivatives	(1)	2	(1)	5
Foreign currency translation adjustments	(17)	6	(19)	(14)
Post-retirement benefit obligation	6	7	17	22
Total other comprehensive (loss) income	(12)	11	(3)	13
Equity in other comprehensive loss of subsidiaries, net of tax	(285)	157	(142)	(38)
Less: Other comprehensive income (loss) attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	(297)	168	(145)	(25)
Comprehensive income attributable to Aon shareholders	(174)	276	363	555
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	332	272	1,005	853
Less: Net income attributable to noncontrolling interests	6	8	26	30
Net income attributable to Aon shareholders	326	264	979	823
Change in fair value of investments	0	(10)	(1)	(1)
Change in fair value of derivatives	(7)	7	14	(17)
Foreign currency translation adjustments	(301)	149	(209)	(63)
Post-retirement benefit obligation	20	17	53	43
Total other comprehensive (loss) income	(288)	163	(143)	(38)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive income (loss) attributable to noncontrolling interests	0	3	(2)	1
Total other comprehensive (loss) income attributable to Aon shareholders	(288)	160	(141)	(39)
Comprehensive income attributable to Aon shareholders	38	424	838	784
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(449)	(372)	(1,487)	(1,403)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(449)	(372)	(1,487)	(1,403)
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 (loss) income	0	0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	585	(327)	286	65
Less: Other comprehensive income (loss) attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	585	(327)	286	65
Comprehensive income attributable to Aon shareholders	$ 136	$ (699)	$ (1,201)	$ (1,338)