Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Condensed Financial Statements, Captions [Line Items]
Net income	$ 313	$ 252	$ 649	$ 524
Less: Net income attributable to noncontrolling interests	9	11	20	22
Net income attributable to Aon shareholders	304	241	629	502
Change in fair value of investments	(2)	13	(1)	13
Change in fair value of derivatives	12	(7)	21	(21)
Foreign currency translation adjustments	74	(58)	90	(232)
Post-retirement benefit obligation	18	18	44	41
Total other comprehensive income (loss)	102	(34)	154	(199)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive income attributable to noncontrolling interests	0	(2)	(2)	(2)
Total other comprehensive income (loss) attributable to Aon shareholders	102	(32)	156	(197)
Comprehensive income attributable to Aon shareholders	406	209	785	305
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	304	241	629	502
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	304	241	629	502
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	102	(32)	156	(197)
Less: Other comprehensive income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	102	(32)	156	(197)
Comprehensive income attributable to Aon shareholders	406	209	785	305
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	199	227	403	473
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	199	227	403	473
Change in fair value of investments	0	4	0	4
Change in fair value of derivatives	(1)	2	0	3
Foreign currency translation adjustments	15	(11)	(2)	(19)
Post-retirement benefit obligation	5	7	11	15
Total other comprehensive income (loss)	19	2	9	3
Equity in other comprehensive loss of subsidiaries, net of tax	73	(34)	142	(195)
Less: Other comprehensive income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	92	(32)	151	(192)
Comprehensive income attributable to Aon shareholders	291	195	554	281
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	334	279	689	580
Less: Net income attributable to noncontrolling interests	9	11	20	22
Net income attributable to Aon shareholders	325	268	669	558
Change in fair value of investments	(2)	9	(1)	9
Change in fair value of derivatives	13	(9)	21	(24)
Foreign currency translation adjustments	59	(47)	92	(213)
Post-retirement benefit obligation	13	11	33	26
Total other comprehensive income (loss)	83	(36)	145	(202)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive income attributable to noncontrolling interests	0	(2)	(2)	(2)
Total other comprehensive income (loss) attributable to Aon shareholders	83	(34)	147	(200)
Comprehensive income attributable to Aon shareholders	408	234	816	358
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(524)	(495)	(1,072)	(1,031)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(524)	(495)	(1,072)	(1,031)
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	(175)	66	(298)	392
Less: Other comprehensive income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(175)	66	(298)	392
Comprehensive income attributable to Aon shareholders	$ (699)	$ (429)	$ (1,370)	$ (639)