Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Condensed Financial Statements, Captions [Line Items]
Net income	$ 778	$ 308	$ 1,083	$ 645
Less: Net income attributable to noncontrolling interests	9	8	23	20
Net income attributable to Aon shareholders	769	300	1,060	625
Change in fair value of financial instruments	4	(4)	2	(11)
Foreign currency translation adjustments	44	(59)	191	(138)
Postretirement benefit obligation	20	51	38	(150)
Total other comprehensive (loss) income	68	(12)	231	(299)
Equity in other comprehensive income of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	(5)	0	(4)	0
Total other comprehensive income (loss) attributable to Aon shareholders	73	(12)	235	(299)
Comprehensive income (loss) attributable to Aon shareholders	842	288	1,295	326
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income	759	304	1,042	629
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	759	304	1,042	629
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	0	(2)	0	(2)
Postretirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	0	(2)	0	(2)
Equity in other comprehensive income of subsidiaries, net of tax	83	(14)	253	(301)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	83	(16)	253	(303)
Comprehensive income (loss) attributable to Aon shareholders	842	288	1,295	326
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income	531	135	713	388
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	531	135	713	388
Change in fair value of financial instruments	2	2	0	0
Foreign currency translation adjustments	0	10	0	21
Postretirement benefit obligation	8	3	16	16
Total other comprehensive (loss) income	10	15	16	37
Equity in other comprehensive income of subsidiaries, net of tax	71	(28)	235	(342)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	81	(13)	251	(305)
Comprehensive income (loss) attributable to Aon shareholders	612	122	964	83
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income	1,514	655	2,227	1,467
Less: Net income attributable to noncontrolling interests	9	8	23	20
Net income attributable to Aon shareholders	1,505	647	2,204	1,447
Change in fair value of financial instruments	2	(6)	2	(11)
Foreign currency translation adjustments	54	(71)	209	(161)
Postretirement benefit obligation	12	48	22	(166)
Total other comprehensive (loss) income	68	(29)	233	(338)
Equity in other comprehensive income of subsidiaries, net of tax	81	(13)	251	(305)
Less: Other comprehensive loss attributable to noncontrolling interests	(5)	0	(4)	0
Total other comprehensive income (loss) attributable to Aon shareholders	154	(42)	488	(643)
Comprehensive income (loss) attributable to Aon shareholders	1,659	605	2,692	804
Consolidation Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income	(2,026)	(786)	(2,899)	(1,839)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(2,026)	(786)	(2,899)	(1,839)
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	(10)	4	(18)	4
Postretirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	(10)	4	(18)	4
Equity in other comprehensive income of subsidiaries, net of tax	(235)	55	(739)	948
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(245)	59	(757)	952
Comprehensive income (loss) attributable to Aon shareholders	$ (2,271)	$ (727)	$ (3,656)	$ (887)