Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net income	$ (12)	$ 192	$ 778	$ 305	$ 459	$ 326	$ 308	$ 337	$ 1,263	$ 1,430	$ 1,422
Less: Net income attributable to noncontrolling interests	7	7	9	14	7	7	8	12	37	34	37
Net income attributable to Aon shareholders	$ (19)	$ 185	$ 769	$ 291	$ 452	$ 319	$ 300	$ 325	1,226	1,396	1,385
Change in fair value of financial instruments											(8)
Net change in fair value of financial instruments									12	(12)	(8)
Foreign currency translation adjustments									390	(495)	(442)
Postretirement benefit obligation									19	16	155
Total other comprehensive income (loss)									421	(491)	(295)
Equity in other comprehensive income (loss) of subsidiaries, net of tax									0	0	0
Less: Other comprehensive income attributable to noncontrolling interests									5	(2)	(6)
Total other comprehensive income (loss) attributable to Aon shareholders									416	(489)	(289)
Comprehensive income attributable to Aon shareholders									1,642	907	1,096
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income									1,208	1,414	1,385
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									1,208	1,414	1,385
Change in fair value of financial instruments											0
Net change in fair value of financial instruments									0	0
Foreign currency translation adjustments									0	(2)	0
Postretirement benefit obligation									0	0	0
Total other comprehensive income (loss)									0	(2)	0
Equity in other comprehensive income (loss) of subsidiaries, net of tax									434	(505)	(289)
Less: Other comprehensive income attributable to noncontrolling interests									0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders									434	(507)	(289)
Comprehensive income attributable to Aon shareholders									1,642	907	1,096
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income									160	379	495
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									160	379	495
Change in fair value of financial instruments											0
Net change in fair value of financial instruments									3	(1)
Foreign currency translation adjustments									0	21	(47)
Postretirement benefit obligation									(101)	68	12
Total other comprehensive income (loss)									(98)	88	(35)
Equity in other comprehensive income (loss) of subsidiaries, net of tax									515	(547)	(259)
Less: Other comprehensive income attributable to noncontrolling interests									0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders									417	(459)	(294)
Comprehensive income attributable to Aon shareholders									577	(80)	201
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income									2,386	2,635	2,310
Less: Net income attributable to noncontrolling interests									37	34	37
Net income attributable to Aon shareholders									2,349	2,601	2,273
Change in fair value of financial instruments											(8)
Net change in fair value of financial instruments									9	(11)
Foreign currency translation adjustments									408	(532)	(395)
Postretirement benefit obligation									120	(52)	143
Total other comprehensive income (loss)									537	(595)	(260)
Equity in other comprehensive income (loss) of subsidiaries, net of tax									417	(459)	(294)
Less: Other comprehensive income attributable to noncontrolling interests									5	(2)	(6)
Total other comprehensive income (loss) attributable to Aon shareholders									949	(1,052)	(548)
Comprehensive income attributable to Aon shareholders									3,298	1,549	1,725
Consolidating Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income									(2,491)	(2,998)	(2,768)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									(2,491)	(2,998)	(2,768)
Change in fair value of financial instruments											0
Net change in fair value of financial instruments									0	0
Foreign currency translation adjustments									(18)	18	0
Postretirement benefit obligation									0	0	0
Total other comprehensive income (loss)									(18)	18	0
Equity in other comprehensive income (loss) of subsidiaries, net of tax									(1,366)	1,511	842
Less: Other comprehensive income attributable to noncontrolling interests									0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders									(1,384)	1,529	842
Comprehensive income attributable to Aon shareholders									$ (3,875)	$ (1,469)	$ (1,926)