Guarantee of Registered Securities 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, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Condensed Financial Statements, Captions [Line Items]
Net income	$ 252	$ 254	$ 524	$ 503
Less: Net income attributable to noncontrolling interests	11	8	22	19
Net income attributable to Aon shareholders	241	246	502	484
Change in fair value of investments	13	0	13	0
Change in fair value of derivatives	(7)	(17)	(21)	(10)
Foreign currency translation adjustments	(58)	(197)	(232)	(93)
Post-retirement benefit obligation	18	18	41	39
Total other comprehensive loss	(34)	(196)	(199)	(64)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	(2)	(3)	(2)	(2)
Total other comprehensive loss attributable to Aon shareholders	(32)	(193)	(197)	(62)
Comprehensive income attributable to Aon shareholders	209	53	305	422
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	241	246	502	484
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	241	246	502	484
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	0	0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	(32)	(193)	(197)	(62)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive loss attributable to Aon shareholders	(32)	(193)	(197)	(62)
Comprehensive income attributable to Aon shareholders	209	53	305	422
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	239	231	493	440
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	239	231	493	440
Change in fair value of investments	4	0	4	0
Change in fair value of derivatives	2	0	3	0
Foreign currency translation adjustments	(11)	(5)	(19)	(1)
Post-retirement benefit obligation	7	9	15	15
Total other comprehensive loss	2	4	3	14
Equity in other comprehensive loss of subsidiaries, net of tax	(34)	(193)	(200)	(77)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive loss attributable to Aon shareholders	(32)	(189)	(197)	(63)
Comprehensive income attributable to Aon shareholders	207	42	296	377
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	518	502	1,073	1,001
Less: Net income attributable to noncontrolling interests	11	8	22	19
Net income attributable to Aon shareholders	507	494	1,051	982
Change in fair value of investments	9	0	9	0
Change in fair value of derivatives	(9)	(17)	(24)	(10)
Foreign currency translation adjustments	(47)	(192)	(213)	(92)
Post-retirement benefit obligation	11	9	26	24
Total other comprehensive loss	(36)	(200)	(202)	(78)
Equity in other comprehensive loss of subsidiaries, net of tax	(32)	(189)	(197)	(63)
Less: Other comprehensive loss attributable to noncontrolling interests	(2)	(3)	(2)	(2)
Total other comprehensive loss attributable to Aon shareholders	(66)	(386)	(397)	(139)
Comprehensive income attributable to Aon shareholders	441	108	654	843
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(746)	(725)	(1,544)	(1,422)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(746)	(725)	(1,544)	(1,422)
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	0	0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	98	575	594	202
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive loss attributable to Aon shareholders	98	575	594	202
Comprehensive income attributable to Aon shareholders	(648)	(150)	(950)	(1,220)
Pro Forma [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	252	254	524	503
Less: Net income attributable to noncontrolling interests	11	8	22	19
Net income attributable to Aon shareholders	241	246	502	484
Total other comprehensive loss	(34)	(196)	(199)	(64)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	(2)	(3)	(2)	(2)
Total other comprehensive loss attributable to Aon shareholders	(32)	(193)	(197)	(62)
Comprehensive income attributable to Aon shareholders	209	53	305	418
Pro Forma [Member] | Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	241	246	502	484
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	241	246	502	484
Total other comprehensive loss	0	0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	(32)	(193)	(197)	(62)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive loss attributable to Aon shareholders	(32)	(193)	(197)	(62)
Comprehensive income attributable to Aon shareholders	209	53	305	422
Pro Forma [Member] | Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	239	231	493	440
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	239	231	493	440
Total other comprehensive loss	2	4	3	14
Equity in other comprehensive loss of subsidiaries, net of tax	(34)	(193)	(200)	(77)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive loss attributable to Aon shareholders	(32)	(189)	(197)	(63)
Comprehensive income attributable to Aon shareholders	207	42	296	373
Pro Forma [Member] | Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	279	271	580	561
Less: Net income attributable to noncontrolling interests	11	8	22	19
Net income attributable to Aon shareholders	268	263	558	542
Total other comprehensive loss	(36)	(200)	(202)	(78)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	(2)	(3)	(2)	(2)
Total other comprehensive loss attributable to Aon shareholders	(34)	(197)	(200)	(76)
Comprehensive income attributable to Aon shareholders	234	66	358	466
Pro Forma [Member] | Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(507)	(494)	(1,051)	(982)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(507)	(494)	(1,051)	(982)
Total other comprehensive loss	0	0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	66	386	397	139
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive loss attributable to Aon shareholders	66	386	397	139
Comprehensive income attributable to Aon shareholders	$ (441)	$ (108)	$ (654)	$ (843)