Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Condensed Financial Statements, Captions [Line Items]
Net income	$ 280	$ 188	$ 607	$ 529
Less: Net income attributable to noncontrolling interests	8	10	20	23
Net income attributable to Aon shareholders	272	178	587	506
Change in fair value of financial instruments	(4)	(6)	(11)	(1)
Foreign currency translation adjustments	(59)	175	(138)	(147)
Post-retirement benefit obligation	51	21	(150)	44
Total other comprehensive (loss) income	(12)	190	(299)	(104)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	0	(1)	0	(2)
Total other comprehensive (loss) income attributable to Aon shareholders	(12)	191	(299)	(102)
Comprehensive income attributable to Aon shareholders	260	369	288	404
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	276	178	591	506
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	276	178	591	506
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	(2)	0	(2)	0
Post-retirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	(2)	0	(2)	0
Equity in other comprehensive loss of subsidiaries, net of tax	(14)	191	(301)	(102)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	(16)	191	(303)	(102)
Comprehensive income attributable to Aon shareholders	260	369	288	404
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	116	64	380	366
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	116	64	380	366
Change in fair value of financial instruments	2	2	0	1
Foreign currency translation adjustments	10	4	21	(19)
Post-retirement benefit obligation	3	9	16	17
Total other comprehensive (loss) income	15	15	37	(1)
Equity in other comprehensive loss of subsidiaries, net of tax	(31)	173	(345)	(94)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	(16)	188	(308)	(95)
Comprehensive income attributable to Aon shareholders	100	252	72	271
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	607	337	1,420	1,096
Less: Net income attributable to noncontrolling interests	8	10	20	23
Net income attributable to Aon shareholders	599	327	1,400	1,073
Change in fair value of financial instruments	(6)	(8)	(11)	(2)
Foreign currency translation adjustments	(71)	171	(161)	(128)
Post-retirement benefit obligation	48	12	(166)	27
Total other comprehensive (loss) income	(29)	175	(338)	(103)
Equity in other comprehensive loss of subsidiaries, net of tax	(16)	188	(308)	(95)
Less: Other comprehensive loss attributable to noncontrolling interests	0	(1)	0	(2)
Total other comprehensive (loss) income attributable to Aon shareholders	(45)	364	(646)	(196)
Comprehensive income attributable to Aon shareholders	554	691	754	877
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(719)	(391)	(1,784)	(1,439)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(719)	(391)	(1,784)	(1,439)
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	4	0	4	0
Post-retirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	4	0	4	0
Equity in other comprehensive loss of subsidiaries, net of tax	61	(552)	954	291
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders	65	(552)	958	291
Comprehensive income attributable to Aon shareholders	$ (654)	$ (943)	$ (826)	$ (1,148)