Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Condensed Financial Statements, Captions [Line Items]
Net income	$ 287	$ 676	$ 58	$ 610	$ 963	$ 668
Less: Net income attributable to noncontrolling interests	10		10		27	26
Net income attributable to Aon shareholders	277		48		936	642
Change in fair value of financial instruments	(8)	7	(1)	14	(1)	13
Foreign currency translation adjustments	(103)	133	(460)	247	30	(213)
Postretirement benefit obligation	14	$ 31	122	$ 48	45	170
Total other comprehensive (loss) income	(97)		(339)		74	(30)
Equity in other comprehensive income (loss) of subsidiaries, net of tax	0		0		0	0
Less: Other comprehensive loss attributable to noncontrolling interests	0		(6)		2	(3)
Total other comprehensive income (loss) attributable to Aon shareholders	(97)		(333)		72	(27)
Comprehensive income (loss) attributable to Aon shareholders	180		(285)		1,008	615
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income	270		66		931	657
Less: Net income attributable to noncontrolling interests	0		0		0	0
Net income attributable to Aon shareholders	270		66		931	657
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	(90)		(351)		77	(42)
Less: Other comprehensive loss attributable to noncontrolling interests	0		0		0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(90)		(351)		77	(42)
Comprehensive income (loss) attributable to Aon shareholders	180		(285)		1,008	615
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income	27		16		540	598
Less: Net income attributable to noncontrolling interests	0		0		0	0
Net income attributable to Aon shareholders	27		16		540	598
Change in fair value of financial instruments	0		(4)		2	(1)
Foreign currency translation adjustments	0		0		0	0
Postretirement benefit obligation	10		11		32	22
Total other comprehensive (loss) income	10		7		34	21
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(118)		(345)		(3)	(60)
Less: Other comprehensive loss attributable to noncontrolling interests	0		0		0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(108)		(338)		31	(39)
Comprehensive income (loss) attributable to Aon shareholders	(81)		(322)		571	559
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income	522		387		1,945	1,826
Less: Net income attributable to noncontrolling interests	10		10		27	26
Net income attributable to Aon shareholders	512		377		1,918	1,800
Change in fair value of financial instruments	(8)		3		(3)	14
Foreign currency translation adjustments	(96)		(478)		35	(228)
Postretirement benefit obligation	4		111		13	148
Total other comprehensive (loss) income	(100)		(364)		45	(66)
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(108)		(338)		31	(39)
Less: Other comprehensive loss attributable to noncontrolling interests	0		(6)		2	(3)
Total other comprehensive income (loss) attributable to Aon shareholders	(208)		(696)		74	(102)
Comprehensive income (loss) attributable to Aon shareholders	304		(319)		1,992	1,698
Consolidation Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income	(532)		(411)		(2,453)	(2,413)
Less: Net income attributable to noncontrolling interests	0		0		0	0
Net income attributable to Aon shareholders	(532)		(411)		(2,453)	(2,413)
Change in fair value of financial instruments	0		0		0	0
Foreign currency translation adjustments	(7)		18		(5)	15
Postretirement benefit obligation	0		0		0	0
Total other comprehensive (loss) income	(7)		18		(5)	15
Equity in other comprehensive income (loss) of subsidiaries, net of tax	316		1,034		(105)	141
Less: Other comprehensive loss attributable to noncontrolling interests	0		0		0	0
Total other comprehensive income (loss) attributable to Aon shareholders	309		1,052		(110)	156
Comprehensive income (loss) attributable to Aon shareholders	$ (223)		$ 641		$ (2,563)	$ (2,257)