Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net income	$ 590	$ 303	$ 188	$ 341	$ 467	$ 315	$ 313	$ 336	$ 1,422	$ 1,431	$ 1,148
Less: Net income attributable to noncontrolling interests	6	8	10	13	8	6	9	11	37	34	35
Net income attributable to Aon shareholders	$ 584	$ 295	$ 178	$ 328	$ 459	$ 309	$ 304	$ 325	1,385	1,397	1,113
Net change in fair value of financial instruments									(8)	4	7
Change in fair value of derivatives											7
Foreign currency translation adjustments									(442)	(507)	(65)
Post-retirement benefit obligation									155	(260)	293
Total other comprehensive (loss) income									(295)	(763)	235
Equity in other comprehensive loss of subsidiaries, net of tax									0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests									(6)	(3)	(1)
Total other comprehensive (loss) income attributable to Aon shareholders									(289)	(760)	236
Comprehensive income attributable to Aon shareholders									1,096	637	1,349
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									1,385	1,397	1,113
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									1,385	1,397	1,113
Net change in fair value of financial instruments									0	0
Change in fair value of derivatives											0
Foreign currency translation adjustments									0	0	0
Post-retirement benefit obligation									0	0	0
Total other comprehensive (loss) income									0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax									(289)	(760)	236
Less: Other comprehensive loss attributable to noncontrolling interests									0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders									(289)	(760)	236
Comprehensive income attributable to Aon shareholders									1,096	637	1,349
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									525	565	815
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									525	565	815
Net change in fair value of financial instruments									0	(3)
Change in fair value of derivatives											5
Foreign currency translation adjustments									(47)	(31)	(60)
Post-retirement benefit obligation									12	(315)	223
Total other comprehensive (loss) income									(35)	(349)	168
Equity in other comprehensive loss of subsidiaries, net of tax									(268)	(411)	69
Less: Other comprehensive loss attributable to noncontrolling interests									0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders									(303)	(760)	237
Comprehensive income attributable to Aon shareholders									222	(195)	1,052
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									2,340	2,153	2,191
Less: Net income attributable to noncontrolling interests									37	34	35
Net income attributable to Aon shareholders									2,303	2,119	2,156
Net change in fair value of financial instruments									(8)	7
Change in fair value of derivatives											2
Foreign currency translation adjustments									(395)	(476)	(5)
Post-retirement benefit obligation									143	55	70
Total other comprehensive (loss) income									(260)	(414)	67
Equity in other comprehensive loss of subsidiaries, net of tax									(303)	(760)	237
Less: Other comprehensive loss attributable to noncontrolling interests									(6)	(3)	(1)
Total other comprehensive (loss) income attributable to Aon shareholders									(557)	(1,171)	305
Comprehensive income attributable to Aon shareholders									1,746	948	2,461
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									(2,828)	(2,684)	(2,971)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									(2,828)	(2,684)	(2,971)
Net change in fair value of financial instruments									0	0
Change in fair value of derivatives											0
Foreign currency translation adjustments									0	0	0
Post-retirement benefit obligation									0	0	0
Total other comprehensive (loss) income									0	0	0
Equity in other comprehensive loss of subsidiaries, net of tax									860	1,931	(542)
Less: Other comprehensive loss attributable to noncontrolling interests									0	0	0
Total other comprehensive (loss) income attributable to Aon shareholders									860	1,931	(542)
Comprehensive income attributable to Aon shareholders									$ (1,968)	$ (753)	$ (3,513)