Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Net income	$ 676	$ 610
Less: Net income attributable to noncontrolling interests	17	16
Net income attributable to Aon shareholders	659	594
Change in fair value of financial instruments	7	14
Foreign currency translation adjustments	133	247
Postretirement benefit obligation	31	48
Total other comprehensive (loss) income	171	309
Equity in other comprehensive income of subsidiaries, net of tax	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	2	3
Total other comprehensive income attributable to Aon shareholders	169	306
Comprehensive income (loss) attributable to Aon shareholders	828	900
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income	661	591
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	661	591
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	167	309
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive income attributable to Aon shareholders	167	309
Comprehensive income (loss) attributable to Aon shareholders	828	900
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income	513	582
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	513	582
Change in fair value of financial instruments	2	3
Foreign currency translation adjustments	0	0
Postretirement benefit obligation	22	11
Total other comprehensive (loss) income	24	14
Equity in other comprehensive income of subsidiaries, net of tax	115	285
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive income attributable to Aon shareholders	139	299
Comprehensive income (loss) attributable to Aon shareholders	652	881
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income	1,423	1,439
Less: Net income attributable to noncontrolling interests	17	16
Net income attributable to Aon shareholders	1,406	1,423
Change in fair value of financial instruments	5	11
Foreign currency translation adjustments	131	250
Postretirement benefit obligation	9	37
Total other comprehensive (loss) income	145	298
Equity in other comprehensive income of subsidiaries, net of tax	139	299
Less: Other comprehensive loss attributable to noncontrolling interests	2	3
Total other comprehensive income attributable to Aon shareholders	282	594
Comprehensive income (loss) attributable to Aon shareholders	1,688	2,017
Consolidation Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income	(1,921)	(2,002)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	(1,921)	(2,002)
Change in fair value of financial instruments	0	0
Foreign currency translation adjustments	2	(3)
Postretirement benefit obligation	0	0
Total other comprehensive (loss) income	2	(3)
Equity in other comprehensive income of subsidiaries, net of tax	(421)	(893)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive income attributable to Aon shareholders	(419)	(896)
Comprehensive income (loss) attributable to Aon shareholders	$ (2,340)	$ (2,898)