Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Financial Statements, Captions [Line Items]
Net income	$ 58	$ 778	$ 668	$ 1,083
Less: Net income attributable to noncontrolling interests	10	9	26	23
Net income attributable to Aon shareholders	48	769	642	1,060
Change in fair value of financial instruments	(1)	4	13	2
Foreign currency translation adjustments	(460)	44	(213)	191
Postretirement benefit obligation	122	20	170	38
Total other comprehensive (loss) income	(339)	68	(30)	231
Equity in other comprehensive income (loss) of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	(6)	(5)	(3)	(4)
Total other comprehensive income (loss) attributable to Aon shareholders	(333)	73	(27)	235
Comprehensive income (loss) attributable to Aon shareholders	(285)	842	615	1,295
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income	66	759	657	1,042
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	66	759	657	1,042
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	(351)	83	(42)	253
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(351)	83	(42)	253
Comprehensive income (loss) attributable to Aon shareholders	(285)	842	615	1,295
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income	15	531	596	713
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	15	531	596	713
Change in fair value of financial instruments	(4)	2	(1)	0
Foreign currency translation adjustments	0	0	0	0
Postretirement benefit obligation	11	8	22	16
Total other comprehensive (loss) income	7	10	21	16
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(345)	71	(60)	235
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(338)	81	(39)	251
Comprehensive income (loss) attributable to Aon shareholders	(323)	612	557	964
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income	386	1,514	1,824	2,227
Less: Net income attributable to noncontrolling interests	10	9	26	23
Net income attributable to Aon shareholders	376	1,505	1,798	2,204
Change in fair value of financial instruments	3	2	14	2
Foreign currency translation adjustments	(478)	54	(228)	209
Postretirement benefit obligation	111	12	148	22
Total other comprehensive (loss) income	(364)	68	(66)	233
Equity in other comprehensive income (loss) of subsidiaries, net of tax	(338)	81	(39)	251
Less: Other comprehensive loss attributable to noncontrolling interests	(6)	(5)	(3)	(4)
Total other comprehensive income (loss) attributable to Aon shareholders	(696)	154	(102)	488
Comprehensive income (loss) attributable to Aon shareholders	(320)	1,659	1,696	2,692
Consolidation Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income	(409)	(2,026)	(2,409)	(2,899)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(409)	(2,026)	(2,409)	(2,899)
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	18	(10)	15	(18)
Postretirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	18	(10)	15	(18)
Equity in other comprehensive income (loss) of subsidiaries, net of tax	1,034	(235)	141	(739)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	1,052	(245)	156	(757)
Comprehensive income (loss) attributable to Aon shareholders	$ 643	$ (2,271)	$ (2,253)	$ (3,656)