Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Condensed Financial Statements, Captions [Line Items]
Net income	$ 153	$ 192	$ 821	$ 1,275
Less: Net income attributable to noncontrolling interests	6	7	32	30
Net income attributable to Aon shareholders	147	185	789	1,245
Change in fair value of financial instruments	1	11	14	13
Foreign currency translation adjustments	(50)	243	(263)	434
Postretirement benefit obligation	(62)	18	108	56
Total other comprehensive (loss) income	(111)	272	(141)	503
Equity in other comprehensive income (loss) of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	(3)	7	(6)	3
Total other comprehensive income (loss) attributable to Aon shareholders	(108)	265	(135)	500
Comprehensive income (loss) attributable to Aon shareholders	39	450	654	1,745
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income	145	185	802	1,227
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	145	185	802	1,227
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	0	0	0	0
Postretirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	0	0	0	0
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(106)	265	(148)	518
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(106)	265	(148)	518
Comprehensive income (loss) attributable to Aon shareholders	39	450	654	1,745
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income	(364)	(216)	232	497
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(364)	(216)	232	497
Change in fair value of financial instruments	(2)	3	(3)	3
Foreign currency translation adjustments	0	0	0	0
Postretirement benefit obligation	12	7	34	23
Total other comprehensive (loss) income	10	10	31	26
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(134)	245	(194)	480
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(124)	255	(163)	506
Comprehensive income (loss) attributable to Aon shareholders	(488)	39	69	1,003
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income	(78)	93	1,746	2,320
Less: Net income attributable to noncontrolling interests	6	7	32	30
Net income attributable to Aon shareholders	(84)	86	1,714	2,290
Change in fair value of financial instruments	3	8	17	10
Foreign currency translation adjustments	(48)	243	(276)	452
Postretirement benefit obligation	(74)	11	74	33
Total other comprehensive (loss) income	(119)	262	(185)	495
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(124)	255	(163)	506
Less: Other comprehensive loss attributable to noncontrolling interests	(3)	7	(6)	3
Total other comprehensive income (loss) attributable to Aon shareholders	(240)	510	(342)	998
Comprehensive income (loss) attributable to Aon shareholders	(324)	596	1,372	3,288
Consolidation Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income	450	130	(1,959)	(2,769)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	450	130	(1,959)	(2,769)
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	(2)	0	13	(18)
Postretirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	(2)	0	13	(18)
Equity in other comprehensive income (loss) of subsidiaries, net of tax	364	(765)	505	(1,504)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	362	(765)	518	(1,522)
Comprehensive income (loss) attributable to Aon shareholders	$ 812	$ (635)	$ (1,441)	$ (4,291)