Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Revenue
Commissions, fees and other	$ 2,842	$ 2,941
Fiduciary investment income	5	6
Total revenue	2,847	2,947
Expenses
Compensation and benefits	1,683	1,751
Other general expenses	723	727
Total operating expenses	2,406	2,478
Operating income	441	469
Interest income	3	2
Interest expense	(65)	(58)
Intercompany interest income (expense)	0	0
Intercompany other (expense) income	0	0
Other income	42	1
Income before income taxes	421	414
Income tax (benefit) expense	80	78
(Loss) income before equity in earnings of subsidiaries	341	336
Equity in earnings of subsidiaries, net of tax	0	0
Net income	341	336
Less: Net income attributable to noncontrolling interests	13	11
Net income attributable to Aon shareholders	328	325
Aon plc [Member]
Revenue
Commissions, fees and other	0	1
Fiduciary investment income	0	0
Total revenue	0	1
Expenses
Compensation and benefits	52	66
Other general expenses	13	4
Total operating expenses	65	70
Operating income	(65)	(69)
Interest income	(5)	(2)
Interest expense	(25)	(9)
Intercompany interest income (expense)	119	111
Intercompany other (expense) income	(50)	(19)
Other income	2	0
Income before income taxes	(24)	12
Income tax (benefit) expense	(5)	3
(Loss) income before equity in earnings of subsidiaries	(19)	9
Equity in earnings of subsidiaries, net of tax	347	316
Net income	328	325
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	328	325
Aon Corporation [Member]
Revenue
Commissions, fees and other	0	0
Fiduciary investment income	0	0
Total revenue	0	0
Expenses
Compensation and benefits	11	7
Other general expenses	2	3
Total operating expenses	13	10
Operating income	(13)	(10)
Interest income	3	0
Interest expense	(34)	(31)
Intercompany interest income (expense)	(105)	(74)
Intercompany other (expense) income	(8)	(15)
Other income	8	0
Income before income taxes	(149)	(130)
Income tax (benefit) expense	(50)	(50)
(Loss) income before equity in earnings of subsidiaries	(99)	(80)
Equity in earnings of subsidiaries, net of tax	403	333
Net income	304	253
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	304	253
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,842	2,940
Fiduciary investment income	5	6
Total revenue	2,847	2,946
Expenses
Compensation and benefits	1,620	1,678
Other general expenses	708	720
Total operating expenses	2,328	2,398
Operating income	519	548
Interest income	5	4
Interest expense	(6)	(18)
Intercompany interest income (expense)	(14)	(37)
Intercompany other (expense) income	58	34
Other income	32	1
Income before income taxes	594	532
Income tax (benefit) expense	135	125
(Loss) income before equity in earnings of subsidiaries	459	407
Equity in earnings of subsidiaries, net of tax	304	253
Net income	763	660
Less: Net income attributable to noncontrolling interests	13	11
Net income attributable to Aon shareholders	750	649
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	(1,054)	(902)
Net income	(1,054)	(902)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	$ (1,054)	$ (902)