Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ 192	$ 326	$ 1,275	$ 971
Net income (loss) before equity in earnings of subsidiaries	192	326	1,275	971
Less: Net income attributable to noncontrolling interests	7	7	30	27
Net income attributable to Aon shareholders	185	319	1,245	944
Change in fair value of financial instruments	11	0	13	(11)
Foreign currency translation adjustments	243	(89)	434	(227)
Postretirement benefit obligation	18	18	56	(132)
Total other comprehensive (loss) income	272	(71)	503	(370)
Equity in other comprehensive income (loss) of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	7	0	3	0
Total other comprehensive income (loss) attributable to Aon shareholders	265	(71)	500	(370)
Comprehensive income (loss) attributable to Aon shareholders	450	248	1,745	574
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	185	316	1,227	945
Net income (loss) before equity in earnings of subsidiaries	221	238	(35)	31
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	185	316	1,227	945
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	0	0	0	(2)
Postretirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	0	0	0	(2)
Equity in other comprehensive income (loss) of subsidiaries, net of tax	265	(68)	518	(369)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	265	(68)	518	(371)
Comprehensive income (loss) attributable to Aon shareholders	450	248	1,745	574
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	(216)	(120)	497	268
Net income (loss) before equity in earnings of subsidiaries	(338)	(345)	(531)	(568)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(216)	(120)	497	268
Change in fair value of financial instruments	3	1	3	1
Foreign currency translation adjustments	0	1	0	22
Postretirement benefit obligation	7	7	23	23
Total other comprehensive (loss) income	10	9	26	46
Equity in other comprehensive income (loss) of subsidiaries, net of tax	245	(83)	480	(425)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	255	(74)	506	(379)
Comprehensive income (loss) attributable to Aon shareholders	39	(194)	1,003	(111)
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	93	310	2,320	1,777
Net income (loss) before equity in earnings of subsidiaries	309	430	1,823	1,509
Less: Net income attributable to noncontrolling interests	7	7	30	27
Net income attributable to Aon shareholders	86	303	2,290	1,750
Change in fair value of financial instruments	8	(1)	10	(12)
Foreign currency translation adjustments	243	(87)	452	(248)
Postretirement benefit obligation	11	11	33	(155)
Total other comprehensive (loss) income	262	(77)	495	(415)
Equity in other comprehensive income (loss) of subsidiaries, net of tax	255	(74)	506	(379)
Less: Other comprehensive loss attributable to noncontrolling interests	7	0	3	0
Total other comprehensive income (loss) attributable to Aon shareholders	510	(151)	998	(794)
Comprehensive income (loss) attributable to Aon shareholders	596	152	3,288	956
Consolidation Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	130	(180)	(2,769)	(2,019)
Net income (loss) before equity in earnings of subsidiaries	0	3	18	(1)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	130	(180)	(2,769)	(2,019)
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	0	(3)	(18)	1
Postretirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	0	(3)	(18)	1
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(765)	225	(1,504)	1,173
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(765)	222	(1,522)	1,174
Comprehensive income (loss) attributable to Aon shareholders	$ (635)	$ 42	$ (4,291)	$ (845)