Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net income	$ 353	$ 153	$ 58	$ 610	$ (12)	$ 192	$ 778	$ 305	$ 1,174	$ 1,263	$ 1,430
Less: Net income attributable to noncontrolling interests	8	6	10	16	7	7	9	14	40	37	34
Net income attributable to Aon shareholders	$ 345	$ 147	$ 48	$ 594	$ (19)	$ 185	$ 769	$ 291	1,134	1,226	1,396
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									11	12	(12)
Foreign currency translation adjustments									(444)	390	(495)
Postretirement benefit obligation									17	19	16
Total other comprehensive income (loss)									(416)	421	(491)
Equity in other comprehensive income (loss) of subsidiaries, net of tax									0	0	0
Less: Other comprehensive income (loss) attributable to noncontrolling interests									(4)	5	(2)
Total other comprehensive income (loss) attributable to Aon shareholders									(412)	416	(489)
Comprehensive income attributable to Aon shareholders									722	1,642	907
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income									1,155	1,208	1,414
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									1,155	1,208	1,414
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									0	0	0
Foreign currency translation adjustments									0	0	(2)
Postretirement benefit obligation									0	0	0
Total other comprehensive income (loss)									0	0	(2)
Equity in other comprehensive income (loss) of subsidiaries, net of tax									(433)	434	(505)
Less: Other comprehensive income (loss) attributable to noncontrolling interests									0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders									(433)	434	(507)
Comprehensive income attributable to Aon shareholders									722	1,642	907
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income									129	227	409
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									129	227	409
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									0	3	(1)
Foreign currency translation adjustments									0	0	21
Postretirement benefit obligation									(2)	(101)	68
Total other comprehensive income (loss)									(2)	(98)	88
Equity in other comprehensive income (loss) of subsidiaries, net of tax									(415)	515	(547)
Less: Other comprehensive income (loss) attributable to noncontrolling interests									0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders									(417)	417	(459)
Comprehensive income attributable to Aon shareholders									(288)	644	(50)
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income									2,299	2,453	2,665
Less: Net income attributable to noncontrolling interests									40	37	34
Net income attributable to Aon shareholders									2,259	2,416	2,631
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									11	9	(11)
Foreign currency translation adjustments									(465)	408	(532)
Postretirement benefit obligation									19	120	(52)
Total other comprehensive income (loss)									(435)	537	(595)
Equity in other comprehensive income (loss) of subsidiaries, net of tax									(417)	417	(459)
Less: Other comprehensive income (loss) attributable to noncontrolling interests									(4)	5	(2)
Total other comprehensive income (loss) attributable to Aon shareholders									(848)	949	(1,052)
Comprehensive income attributable to Aon shareholders									1,411	3,365	1,579
Consolidating Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income									(2,409)	(2,625)	(3,058)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									(2,409)	(2,625)	(3,058)
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									0	0	0
Foreign currency translation adjustments									21	(18)	18
Postretirement benefit obligation									0	0	0
Total other comprehensive income (loss)									21	(18)	18
Equity in other comprehensive income (loss) of subsidiaries, net of tax									1,265	(1,366)	1,511
Less: Other comprehensive income (loss) attributable to noncontrolling interests									0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders									1,286	(1,384)	1,529
Comprehensive income attributable to Aon shareholders									$ (1,123)	$ (4,009)	$ (1,529)