Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net income	$ 459	$ 326	$ 308	$ 337	$ 540	$ 311	$ 214	$ 357	$ 645	$ 971	$ 1,430	$ 1,422	$ 1,431
Less: Net income attributable to noncontrolling interests	7	7	8	12	6	8	10	13			34	37	34
Net income attributable to Aon shareholders	$ 452	$ 319	$ 300	$ 325	$ 534	$ 303	$ 204	$ 344			1,396	1,385	1,397
Change in fair value of financial instruments													4
Net change in fair value of financial instruments											(12)	(8)	4
Foreign currency translation adjustments											(495)	(442)	(507)
Post-retirement benefit obligation											16	155	(260)
Total other comprehensive loss											(491)	(295)	(763)
Equity in other comprehensive loss of subsidiaries, net of tax											0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests											(2)	(6)	(3)
Total other comprehensive loss attributable to Aon shareholders											(489)	(289)	(760)
Comprehensive income attributable to Aon shareholders											907	1,096	637
Aon plc
Condensed Financial Statements, Captions [Line Items]
Net income											1,414	1,385	1,397
Less: Net income attributable to noncontrolling interests											0	0	0
Net income attributable to Aon shareholders											1,414	1,385	1,397
Change in fair value of financial instruments													0
Net change in fair value of financial instruments											0	0
Foreign currency translation adjustments											(2)	0	0
Post-retirement benefit obligation											0	0	0
Total other comprehensive loss											(2)	0	0
Equity in other comprehensive loss of subsidiaries, net of tax											(505)	(289)	(760)
Less: Other comprehensive loss attributable to noncontrolling interests											0	0	0
Total other comprehensive loss attributable to Aon shareholders											(507)	(289)	(760)
Comprehensive income attributable to Aon shareholders											907	1,096	637
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
Net income											379	496	546
Less: Net income attributable to noncontrolling interests											0	0	0
Net income attributable to Aon shareholders											379	496	546
Change in fair value of financial instruments													(3)
Net change in fair value of financial instruments											(1)	0
Foreign currency translation adjustments											21	(47)	(31)
Post-retirement benefit obligation											68	12	(315)
Total other comprehensive loss											88	(35)	(349)
Equity in other comprehensive loss of subsidiaries, net of tax											(547)	(259)	(409)
Less: Other comprehensive loss attributable to noncontrolling interests											0	0	0
Total other comprehensive loss attributable to Aon shareholders											(459)	(294)	(758)
Comprehensive income attributable to Aon shareholders											(80)	202	(212)
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income											2,635	2,311	2,134
Less: Net income attributable to noncontrolling interests											34	37	34
Net income attributable to Aon shareholders											2,601	2,274	2,100
Change in fair value of financial instruments													7
Net change in fair value of financial instruments											(11)	(8)
Foreign currency translation adjustments											(532)	(395)	(476)
Post-retirement benefit obligation											(52)	143	55
Total other comprehensive loss											(595)	(260)	(414)
Equity in other comprehensive loss of subsidiaries, net of tax											(459)	(294)	(758)
Less: Other comprehensive loss attributable to noncontrolling interests											(2)	(6)	(3)
Total other comprehensive loss attributable to Aon shareholders											(1,052)	(548)	(1,169)
Comprehensive income attributable to Aon shareholders											1,549	1,726	931
Consolidating Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income											(2,998)	(2,770)	(2,646)
Less: Net income attributable to noncontrolling interests											0	0	0
Net income attributable to Aon shareholders											(2,998)	(2,770)	(2,646)
Change in fair value of financial instruments													0
Net change in fair value of financial instruments											0	0
Foreign currency translation adjustments											18	0	0
Post-retirement benefit obligation											0	0	0
Total other comprehensive loss											18	0	0
Equity in other comprehensive loss of subsidiaries, net of tax											1,511	842	1,927
Less: Other comprehensive loss attributable to noncontrolling interests											0	0	0
Total other comprehensive loss attributable to Aon shareholders											1,529	842	1,927
Comprehensive income attributable to Aon shareholders											$ (1,469)	$ (1,928)	$ (719)