Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Condensed Financial Statements, Captions [Line Items]
Net income	$ 228	$ 287	$ 676	$ 153	$ 58	$ 610	$ 1,191	$ 821
Less: Net income attributable to noncontrolling interests	6			6			33	32
Net income attributable to Aon shareholders	222			147			1,158	789
Change in fair value of financial instruments	(2)	(8)	7	1	(1)	14	(3)	14
Foreign currency translation adjustments	(212)	(103)	133	(50)	(460)	247	(182)	(263)
Postretirement benefit obligation	17	$ 14	$ 31	(62)	$ 122	$ 48	62	108
Net current period other comprehensive income (loss)	(197)			(111)			(123)	(141)
Equity in other comprehensive income (loss) of subsidiaries, net of tax	0			0			0	0
Less: Other comprehensive loss attributable to noncontrolling interests	(4)			(3)			(2)	(6)
Total other comprehensive loss attributable to Aon shareholders	(193)			(108)			(121)	(135)
Comprehensive income (loss) attributable to Aon shareholders	29			39			1,037	654
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income	244			145			1,175	802
Less: Net income attributable to noncontrolling interests	0			0			0	0
Net income attributable to Aon shareholders	244			145			1,175	802
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
Net current period other comprehensive income (loss)	0			0			0	0
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(215)			(106)			(138)	(148)
Less: Other comprehensive loss attributable to noncontrolling interests	0			0			0	0
Total other comprehensive loss attributable to Aon shareholders	(215)			(106)			(138)	(148)
Comprehensive income (loss) attributable to Aon shareholders	29			39			1,037	654
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income	(36)			(364)			504	234
Less: Net income attributable to noncontrolling interests	0			0			0	0
Net income attributable to Aon shareholders	(36)			(364)			504	234
Change in fair value of financial instruments	0			(2)			2	(3)
Foreign currency translation adjustments	0			0			0	0
Postretirement benefit obligation	10			12			42	34
Net current period other comprehensive income (loss)	10			10			44	31
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(220)			(134)			(223)	(194)
Less: Other comprehensive loss attributable to noncontrolling interests	0			0			0	0
Total other comprehensive loss attributable to Aon shareholders	(210)			(124)			(179)	(163)
Comprehensive income (loss) attributable to Aon shareholders	(246)			(488)			325	71
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income	443			(78)			2,388	1,748
Less: Net income attributable to noncontrolling interests	6			6			33	32
Net income attributable to Aon shareholders	437			(84)			2,355	1,716
Change in fair value of financial instruments	(2)			3			(5)	17
Foreign currency translation adjustments	(234)			(48)			(199)	(276)
Postretirement benefit obligation	7			(74)			20	74
Net current period other comprehensive income (loss)	(229)			(119)			(184)	(185)
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(210)			(124)			(179)	(163)
Less: Other comprehensive loss attributable to noncontrolling interests	(4)			(3)			(2)	(6)
Total other comprehensive loss attributable to Aon shareholders	(435)			(240)			(361)	(342)
Comprehensive income (loss) attributable to Aon shareholders	2			(324)			1,994	1,374
Consolidation Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income	(423)			450			(2,876)	(1,963)
Less: Net income attributable to noncontrolling interests	0			0			0	0
Net income attributable to Aon shareholders	(423)			450			(2,876)	(1,963)
Change in fair value of financial instruments	0			0			0	0
Foreign currency translation adjustments	22			(2)			17	13
Postretirement benefit obligation	0			0			0	0
Net current period other comprehensive income (loss)	22			(2)			17	13
Equity in other comprehensive income (loss) of subsidiaries, net of tax	645			364			540	505
Less: Other comprehensive loss attributable to noncontrolling interests	0			0			0	0
Total other comprehensive loss attributable to Aon shareholders	667			362			557	518
Comprehensive income (loss) attributable to Aon shareholders	$ 244			$ 812			$ (2,319)	$ (1,445)