Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Condensed Financial Statements, Captions [Line Items]
Net income	$ 188	$ 313	$ 529	$ 649
Less: Net income attributable to noncontrolling interests	10	9	23	20
Net income attributable to Aon shareholders	178	304	506	629
Change in fair value of financial instruments	(6)	10	(1)	20
Foreign currency translation adjustments	175	74	(147)	90
Post-retirement benefit obligation	21	18	44	44
Total other comprehensive (loss) income	190	102	(104)	154
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive (loss) income attributable to noncontrolling interests	(1)	0	(2)	(2)
Total other comprehensive (loss) income attributable to Aon shareholders	191	102	(102)	156
Comprehensive income attributable to Aon shareholders	369	406	404	785
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	178	304	506	629
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	178	304	506	629
Change in fair value of financial instruments	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	191	102	(102)	156
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	191	102	(102)	156
Comprehensive income attributable to Aon shareholders	369	406	404	785
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	64	220	366	475
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	64	220	366	475
Change in fair value of financial instruments	2	(1)	1	0
Foreign currency translation adjustments	4	15	(19)	(2)
Post-retirement benefit obligation	9	5	17	11
Total other comprehensive (loss) income	15	19	(1)	9
Equity in other comprehensive loss of subsidiaries, net of tax	173	73	(94)	142
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	188	92	(95)	151
Comprehensive income attributable to Aon shareholders	252	312	271	626
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	337	575	1,096	1,237
Less: Net income attributable to noncontrolling interests	10	9	23	20
Net income attributable to Aon shareholders	327	566	1,073	1,217
Change in fair value of financial instruments	(8)	11	(2)	20
Foreign currency translation adjustments	171	59	(128)	92
Post-retirement benefit obligation	12	13	27	33
Total other comprehensive (loss) income	175	83	(103)	145
Equity in other comprehensive loss of subsidiaries, net of tax	188	0	(95)	0
Less: Other comprehensive (loss) income attributable to noncontrolling interests	(1)	0	(2)	(2)
Total other comprehensive (loss) income attributable to Aon shareholders	364	83	(196)	147
Comprehensive income attributable to Aon shareholders	691	649	877	1,364
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(391)	(786)	(1,439)	(1,692)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(391)	(786)	(1,439)	(1,692)
Change in fair value of financial instruments	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	(552)	(175)	291	(298)
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	(552)	(175)	291	(298)
Comprehensive income attributable to Aon shareholders	$ (943)	$ (961)	$ (1,148)	$ (1,990)