Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ 314	$ 303	$ 921	$ 832
Less: Net income attributable to noncontrolling interests	7	8	27	31
Net income attributable to Aon shareholders	307	295	894	801
Change in fair value of financial instruments	0	(10)	(11)	(11)
Foreign currency translation adjustments	(89)	(229)	(227)	(376)
Post-retirement benefit obligation	18	18	(132)	62
Total other comprehensive (loss) income	(71)	(221)	(370)	(325)
Equity in other comprehensive loss of subsidiaries, net of tax	0	0	0	0
Less: Other comprehensive loss attributable to noncontrolling interests	0	(4)	0	(6)
Total other comprehensive loss attributable to Aon shareholders	(71)	(217)	(370)	(319)
Comprehensive income (loss) attributable to Aon shareholders	236	78	524	482
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	304	295	895	801
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	304	295	895	801
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	0	0	(2)	0
Post-retirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	0	0	(2)	0
Equity in other comprehensive loss of subsidiaries, net of tax	(68)	(217)	(369)	(319)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive loss attributable to Aon shareholders	(68)	(217)	(371)	(319)
Comprehensive income (loss) attributable to Aon shareholders	236	78	524	482
Aon Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	(136)	234	244	600
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(136)	234	244	600
Change in fair value of financial instruments	1	(1)	1	0
Foreign currency translation adjustments	1	(24)	22	(43)
Post-retirement benefit obligation	7	8	23	25
Total other comprehensive (loss) income	9	(17)	46	(18)
Equity in other comprehensive loss of subsidiaries, net of tax	(81)	(196)	(426)	(290)
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive loss attributable to Aon shareholders	(72)	(213)	(380)	(308)
Comprehensive income (loss) attributable to Aon shareholders	(208)	21	(136)	292
Other Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	283	662	1,703	1,757
Less: Net income attributable to noncontrolling interests	7	8	27	31
Net income attributable to Aon shareholders	276	654	1,676	1,726
Change in fair value of financial instruments	(1)	(9)	(12)	(11)
Foreign currency translation adjustments	(87)	(205)	(248)	(333)
Post-retirement benefit obligation	11	10	(155)	37
Total other comprehensive (loss) income	(77)	(204)	(415)	(307)
Equity in other comprehensive loss of subsidiaries, net of tax	(72)	(213)	(380)	(308)
Less: Other comprehensive loss attributable to noncontrolling interests	0	(4)	0	(6)
Total other comprehensive loss attributable to Aon shareholders	(149)	(413)	(795)	(609)
Comprehensive income (loss) attributable to Aon shareholders	127	241	881	1,117
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	(137)	(888)	(1,921)	(2,326)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	(137)	(888)	(1,921)	(2,326)
Change in fair value of financial instruments	0	0	0	0
Foreign currency translation adjustments	(3)	0	1	0
Post-retirement benefit obligation	0	0	0	0
Total other comprehensive (loss) income	(3)	0	1	0
Equity in other comprehensive loss of subsidiaries, net of tax	221	626	1,175	917
Less: Other comprehensive loss attributable to noncontrolling interests	0	0	0	0
Total other comprehensive loss attributable to Aon shareholders	218	626	1,176	917
Comprehensive income (loss) attributable to Aon shareholders	$ 81	$ (262)	$ (745)	$ (1,409)