Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net income	$ 610	$ 305
Less: Net income attributable to noncontrolling interests	16	14
Net income attributable to Aon shareholders	594	291
Change in fair value of financial instruments	14	(2)
Foreign currency translation adjustments	247	147
Postretirement benefit obligation	48	18
Total other comprehensive (loss) income	309	163
Equity in other comprehensive income of subsidiaries, net of tax	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	3	1
Total other comprehensive income attributable to Aon shareholders	306	162
Comprehensive income (loss) attributable to Aon shareholders	900	453
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income	591	283
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	591	283
Change in fair value of financial instruments	0	0
Foreign currency translation adjustments	0	0
Postretirement benefit obligation	0	0
Total other comprehensive (loss) income	0	0
Equity in other comprehensive income of subsidiaries, net of tax	309	170
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive income attributable to Aon shareholders	309	170
Comprehensive income (loss) attributable to Aon shareholders	900	453
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income	581	182
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	581	182
Change in fair value of financial instruments	3	(2)
Foreign currency translation adjustments	0	0
Postretirement benefit obligation	11	8
Total other comprehensive (loss) income	14	6
Equity in other comprehensive income of subsidiaries, net of tax	285	164
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive income attributable to Aon shareholders	299	170
Comprehensive income (loss) attributable to Aon shareholders	880	352
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income	1,438	713
Less: Net income attributable to noncontrolling interests	16	14
Net income attributable to Aon shareholders	1,422	699
Change in fair value of financial instruments	11	0
Foreign currency translation adjustments	250	155
Postretirement benefit obligation	37	10
Total other comprehensive (loss) income	298	165
Equity in other comprehensive income of subsidiaries, net of tax	299	170
Less: Other comprehensive loss attributable to noncontrolling interests	3	1
Total other comprehensive income attributable to Aon shareholders	594	334
Comprehensive income (loss) attributable to Aon shareholders	2,016	1,033
Consolidation Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income	(2,000)	(873)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	(2,000)	(873)
Change in fair value of financial instruments	0	0
Foreign currency translation adjustments	(3)	(8)
Postretirement benefit obligation	0	0
Total other comprehensive (loss) income	(3)	(8)
Equity in other comprehensive income of subsidiaries, net of tax	(893)	(504)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive income attributable to Aon shareholders	(896)	(512)
Comprehensive income (loss) attributable to Aon shareholders	$ (2,896)	$ (1,385)