Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Revenue
Commissions, fees and other	$ 2,787	$ 2,842
Fiduciary investment income	5	5
Total revenue	2,792	2,847
Expenses
Compensation and benefits	1,649	1,683
Other general expenses	693	723
Total operating expenses	2,342	2,406
Operating income	450	441
Interest income	2	3
Interest expense	(69)	(65)
Intercompany interest income (expense)	0	0
Intercompany other (expense) income	0	0
Other income	18	42
Income before income taxes	401	421
Income tax (benefit) expense	74	80
(Loss) income before equity in earnings of subsidiaries	327	341
Equity in earnings of subsidiaries, net of tax	0	0
Net income	327	341
Less: Net income attributable to noncontrolling interests	12	13
Net income attributable to Aon shareholders	315	328
Aon plc [Member]
Revenue
Commissions, fees and other	0	0
Fiduciary investment income	0	0
Total revenue	0	0
Expenses
Compensation and benefits	43	52
Other general expenses	7	13
Total operating expenses	50	65
Operating income	(50)	(65)
Interest income	(7)	(5)
Interest expense	(38)	(25)
Intercompany interest income (expense)	4	119
Intercompany other (expense) income	(54)	(50)
Other income	0	2
Income before income taxes	(145)	(24)
Income tax (benefit) expense	(26)	(5)
(Loss) income before equity in earnings of subsidiaries	(119)	(19)
Equity in earnings of subsidiaries, net of tax	434	347
Net income	315	328
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	315	328
Aon Corporation [Member]
Revenue
Commissions, fees and other	0	0
Fiduciary investment income	0	0
Total revenue	0	0
Expenses
Compensation and benefits	3	11
Other general expenses	2	2
Total operating expenses	5	13
Operating income	(5)	(13)
Interest income	5	3
Interest expense	(28)	(34)
Intercompany interest income (expense)	(133)	(105)
Intercompany other (expense) income	1	(8)
Other income	(5)	8
Income before income taxes	(165)	(149)
Income tax (benefit) expense	(62)	(50)
(Loss) income before equity in earnings of subsidiaries	(103)	(99)
Equity in earnings of subsidiaries, net of tax	367	403
Net income	264	304
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	264	304
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,787	2,842
Fiduciary investment income	5	5
Total revenue	2,792	2,847
Expenses
Compensation and benefits	1,603	1,620
Other general expenses	684	708
Total operating expenses	2,287	2,328
Operating income	505	519
Interest income	4	5
Interest expense	(3)	(6)
Intercompany interest income (expense)	129	(14)
Intercompany other (expense) income	53	58
Other income	23	32
Income before income taxes	711	594
Income tax (benefit) expense	162	135
(Loss) income before equity in earnings of subsidiaries	549	459
Equity in earnings of subsidiaries, net of tax	264	304
Net income	813	763
Less: Net income attributable to noncontrolling interests	12	13
Net income attributable to Aon shareholders	801	750
Consolidating Adjustments [Member]
Revenue
Commissions, fees and other	0	0
Fiduciary investment income	0	0
Total revenue	0	0
Expenses
Compensation and benefits	0	0
Other general expenses	0	0
Total operating expenses	0	0
Operating income	0	0
Interest income	0	0
Interest expense	0	0
Intercompany interest income (expense)	0	0
Intercompany other (expense) income	0	0
Other income	0	0
Income before income taxes	0	0
Income tax (benefit) expense	0	0
(Loss) income before equity in earnings of subsidiaries	0	0
Equity in earnings of subsidiaries, net of tax	(1,065)	(1,054)
Net income	(1,065)	(1,054)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	$ (1,065)	$ (1,054)