Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net income	$ 327	$ 341
Less: Net income attributable to noncontrolling interests	12	13
Net income attributable to Aon shareholders	315	328
Change in fair value of financial instruments	(7)	5
Foreign currency translation adjustments	(79)	(322)
Post-retirement benefit obligation	(201)	23
Total other comprehensive (loss)	(287)	(294)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	(1)
Total other comprehensive (loss) attributable to Aon shareholders	(287)	(293)
Comprehensive income attributable to Aon shareholders	28	35
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	315	328
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	315	328
Change in fair value of financial instruments	0	0
Foreign currency translation adjustments	0	0
Post-retirement benefit obligation	0	0
Total other comprehensive (loss)	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	(287)	(293)
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0
Total other comprehensive (loss) attributable to Aon shareholders	(287)	(293)
Comprehensive income attributable to Aon shareholders	28	35
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	264	304
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	264	304
Change in fair value of financial instruments	(2)	(1)
Foreign currency translation adjustments	11	(23)
Post-retirement benefit obligation	13	8
Total other comprehensive (loss)	22	(16)
Equity in other comprehensive loss of subsidiaries, net of tax	(314)	(267)
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0
Total other comprehensive (loss) attributable to Aon shareholders	(292)	(283)
Comprehensive income attributable to Aon shareholders	(28)	21
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	813	763
Less: Net income attributable to noncontrolling interests	12	13
Net income attributable to Aon shareholders	801	750
Change in fair value of financial instruments	(5)	6
Foreign currency translation adjustments	(90)	(299)
Post-retirement benefit obligation	(214)	15
Total other comprehensive (loss)	(309)	(278)
Equity in other comprehensive loss of subsidiaries, net of tax	(292)	(283)
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	(1)
Total other comprehensive (loss) attributable to Aon shareholders	(601)	(560)
Comprehensive income attributable to Aon shareholders	200	190
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(1,065)	(1,054)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	(1,065)	(1,054)
Change in fair value of financial instruments	0	0
Foreign currency translation adjustments	0	0
Post-retirement benefit obligation	0	0
Total other comprehensive (loss)	0	0
Equity in other comprehensive loss of subsidiaries, net of tax	893	843
Less: Other comprehensive (loss) income attributable to noncontrolling interests	0	0
Total other comprehensive (loss) attributable to Aon shareholders	893	843
Comprehensive income attributable to Aon shareholders	$ (172)	$ (211)