Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Revenue
Commissions, fees and other	$ 2,761	$ 2,800	$ 5,548	$ 5,642
Fiduciary investment income	5	5	10	10
Total revenue	2,766	2,805	5,558	5,652
Expenses
Compensation and benefits	1,688	1,653	3,337	3,336
Other general expenses	673	875	1,366	1,598
Total operating expenses	2,361	2,528	4,703	4,934
Operating income	405	277	855	718
Interest income	3	4	5	7
Interest expense	(73)	(68)	(142)	(133)
Intercompany interest income (expense)	0	0	0	0
Intercompany other (expense) income	0	0	0	0
Other income	0	1	18	43
Income before income taxes	335	214	736	635
Income tax (benefit) expense	55	26	129	106
(Loss) income before equity in earnings of subsidiaries	280	188	607	529
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	280	188	607	529
Less: Net income attributable to noncontrolling interests	8	10	20	23
Net income attributable to Aon shareholders	272	178	587	506
Aon plc [Member]
Revenue
Commissions, fees and other	0	0	0	0
Fiduciary investment income	0	0	0	0
Total revenue	0	0	0	0
Expenses
Compensation and benefits	8	5	51	57
Other general expenses	(1)	(4)	6	9
Total operating expenses	7	1	57	66
Operating income	(7)	(1)	(57)	(66)
Interest income	0	(5)	0	(9)
Interest expense	(49)	(29)	(94)	(54)
Intercompany interest income (expense)	3	119	7	239
Intercompany other (expense) income	(57)	(56)	(111)	(106)
Other income	2	(1)	2	1
Income before income taxes	(108)	27	(253)	5
Income tax (benefit) expense	(21)	6	(47)	1
(Loss) income before equity in earnings of subsidiaries	(87)	21	(206)	4
Equity in earnings of subsidiaries, net of tax	363	157	797	502
Net income	276	178	591	506
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	276	178	591	506
Aon Corporation [Member]
Revenue
Commissions, fees and other	0	0	0	0
Fiduciary investment income	0	0	0	0
Total revenue	0	0	0	0
Expenses
Compensation and benefits	3	10	6	21
Other general expenses	2	2	4	4
Total operating expenses	5	12	10	25
Operating income	(5)	(12)	(10)	(25)
Interest income	4	4	9	7
Interest expense	(26)	(33)	(54)	(68)
Intercompany interest income (expense)	(137)	(116)	(270)	(221)
Intercompany other (expense) income	(16)	(15)	(15)	(23)
Other income	(4)	0	(9)	8
Income before income taxes	(184)	(172)	(349)	(322)
Income tax (benefit) expense	(64)	(66)	(126)	(117)
(Loss) income before equity in earnings of subsidiaries	(120)	(106)	(223)	(205)
Equity in earnings of subsidiaries, net of tax	236	170	603	571
Net income	116	64	380	366
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	116	64	380	366
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,761	2,800	5,548	5,642
Fiduciary investment income	5	5	10	10
Total revenue	2,766	2,805	5,558	5,652
Expenses
Compensation and benefits	1,677	1,638	3,280	3,258
Other general expenses	672	877	1,356	1,585
Total operating expenses	2,349	2,515	4,636	4,843
Operating income	417	290	922	809
Interest income	5	5	9	9
Interest expense	(4)	(6)	(7)	(11)
Intercompany interest income (expense)	134	(3)	263	(18)
Intercompany other (expense) income	73	71	126	129
Other income	6	2	29	34
Income before income taxes	631	359	1,342	952
Income tax (benefit) expense	140	86	302	222
(Loss) income before equity in earnings of subsidiaries	491	273	1,040	730
Equity in earnings of subsidiaries, net of tax	116	64	380	366
Net income	607	337	1,420	1,096
Less: Net income attributable to noncontrolling interests	8	10	20	23
Net income attributable to Aon shareholders	599	327	1,400	1,073
Consolidating Adjustments [Member]
Revenue
Commissions, fees and other	0	0	0	0
Fiduciary investment income	0	0	0	0
Total revenue	0	0	0	0
Expenses
Compensation and benefits	0	0	0	0
Other general expenses	0	0	0	0
Total operating expenses	0	0	0	0
Operating income	0	0	0	0
Interest income	(6)	0	(13)	0
Interest expense	6	0	13	0
Intercompany interest income (expense)	0	0	0	0
Intercompany other (expense) income	0	0	0	0
Other income	(4)	0	(4)	0
Income before income taxes	(4)	0	(4)	0
Income tax (benefit) expense	0	0	0	0
(Loss) income before equity in earnings of subsidiaries	(4)	0	(4)	0
Equity in earnings of subsidiaries, net of tax	(715)	(391)	(1,780)	(1,439)
Net income	(719)	(391)	(1,784)	(1,439)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ (719)	$ (391)	$ (1,784)	$ (1,439)