Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ 305	$ 337
Less: Net income attributable to noncontrolling interests	14	12
Net income attributable to Aon shareholders	291	325
Change in fair value of financial instruments	(2)	(7)
Foreign currency translation adjustments	147	(79)
Postretirement benefit obligation	18	(201)
Total other comprehensive (loss) income	163	(287)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	1	0
Total other comprehensive income (loss) attributable to Aon shareholders	162	(287)
Comprehensive income (loss) attributable to Aon shareholders	453	38
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	283	325
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	283	325
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 loss of subsidiaries, net of tax	170	(287)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	170	(287)
Comprehensive income (loss) attributable to Aon shareholders	453	38
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	182	253
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	182	253
Change in fair value of financial instruments	(2)	(2)
Foreign currency translation adjustments	0	11
Postretirement benefit obligation	8	13
Total other comprehensive (loss) income	6	22
Equity in other comprehensive loss of subsidiaries, net of tax	164	(314)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	170	(292)
Comprehensive income (loss) attributable to Aon shareholders	352	(39)
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	713	812
Less: Net income attributable to noncontrolling interests	14	12
Net income attributable to Aon shareholders	699	800
Change in fair value of financial instruments	0	(5)
Foreign currency translation adjustments	155	(90)
Postretirement benefit obligation	10	(214)
Total other comprehensive (loss) income	165	(309)
Equity in other comprehensive loss of subsidiaries, net of tax	170	(292)
Less: Other comprehensive loss attributable to noncontrolling interests	1	0
Total other comprehensive income (loss) attributable to Aon shareholders	334	(601)
Comprehensive income (loss) attributable to Aon shareholders	1,033	199
Elimination
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	(873)	(1,053)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	(873)	(1,053)
Change in fair value of financial instruments	0	0
Foreign currency translation adjustments	(8)	0
Postretirement benefit obligation	0	0
Total other comprehensive (loss) income	(8)	0
Equity in other comprehensive loss of subsidiaries, net of tax	(504)	893
Less: Other comprehensive loss attributable to noncontrolling interests	0	0
Total other comprehensive income (loss) attributable to Aon shareholders	(512)	893
Comprehensive income (loss) attributable to Aon shareholders	$ (1,385)	$ (160)