Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net income	$ 336	$ 272
Less: Net income attributable to noncontrolling interests	11	11
Net income attributable to Aon shareholders	325	261
Change in fair value of investments	1	0
Change in fair value of derivatives	9	(14)
Foreign currency translation adjustments	16	(174)
Post-retirement benefit obligation	26	23
Total other comprehensive income (loss)	52	(165)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0
Less: Other comprehensive income attributable to noncontrolling interests	(2)	0
Total other comprehensive income (loss) attributable to Aon shareholders	54	(165)
Comprehensive income attributable to Aon shareholders	379	96
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	325	261
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	325	261
Change in fair value of investments	0	0
Change in fair value of derivatives	0	0
Foreign currency translation adjustments	0	0
Post-retirement benefit obligation	0	0
Total other comprehensive income (loss)	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	54	(165)
Less: Other comprehensive income attributable to noncontrolling interests	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	54	(165)
Comprehensive income attributable to Aon shareholders	379	96
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	200	247
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	200	247
Change in fair value of investments	0	0
Change in fair value of derivatives	1	1
Foreign currency translation adjustments	(17)	(8)
Post-retirement benefit obligation	6	7
Total other comprehensive income (loss)	(10)	0
Equity in other comprehensive loss of subsidiaries, net of tax	69	(164)
Less: Other comprehensive income attributable to noncontrolling interests	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	59	(164)
Comprehensive income attributable to Aon shareholders	259	83
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	354	300
Less: Net income attributable to noncontrolling interests	11	11
Net income attributable to Aon shareholders	343	289
Change in fair value of investments	1	0
Change in fair value of derivatives	8	(15)
Foreign currency translation adjustments	33	(166)
Post-retirement benefit obligation	20	16
Total other comprehensive income (loss)	62	(165)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0
Less: Other comprehensive income attributable to noncontrolling interests	(2)	0
Total other comprehensive income (loss) attributable to Aon shareholders	64	(165)
Comprehensive income attributable to Aon shareholders	407	124
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(543)	(536)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	(543)	(536)
Change in fair value of investments	0	0
Change in fair value of derivatives	0	0
Foreign currency translation adjustments	0	0
Post-retirement benefit obligation	0	0
Total other comprehensive income (loss)	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	(123)	329
Less: Other comprehensive income attributable to noncontrolling interests	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(123)	329
Comprehensive income attributable to Aon shareholders	$ (666)	$ (207)