Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Revenue
Commissions, fees and other	$ 2,941	$ 2,908
Fiduciary investment income	6	7
Total revenue	2,947	2,915
Expenses
Compensation and benefits	1,751	1,725
Other general expenses	727	780
Total operating expenses	2,478	2,505
Operating income	469	410
Interest income	2	1
Interest expense	(58)	(52)
Intercompany interest income (expense)	0	0
Other income	1	9
Income before income taxes	414	368
Income tax (benefit) expense	78	96
Income (loss) before equity in earnings of subsidiaries	336	272
Equity in earnings of subsidiaries, net of tax	0	0
Net income	336	272
Less: Net income attributable to noncontrolling interests	11	11
Net income attributable to Aon shareholders	325	261
Aon plc [Member]
Revenue
Commissions, fees and other	1	1
Fiduciary investment income	0	0
Total revenue	1	1
Expenses
Compensation and benefits	12	11
Other general expenses	7	7
Total operating expenses	19	18
Operating income	(18)	(17)
Interest income	(2)	0
Interest expense	(9)	(2)
Intercompany interest income (expense)	111	(7)
Other income	0	0
Income before income taxes	82	(26)
Income tax (benefit) expense	20	(5)
Income (loss) before equity in earnings of subsidiaries	62	(21)
Equity in earnings of subsidiaries, net of tax	263	282
Net income	325	261
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	325	261
Aon Corporation [Member]
Revenue
Commissions, fees and other	0	0
Fiduciary investment income	0	0
Total revenue	0	0
Expenses
Compensation and benefits	8	15
Other general expenses	17	6
Total operating expenses	25	21
Operating income	(25)	(21)
Interest income	0	1
Interest expense	(31)	(34)
Intercompany interest income (expense)	(74)	43
Other income	0	(1)
Income before income taxes	(130)	(12)
Income tax (benefit) expense	(50)	(5)
Income (loss) before equity in earnings of subsidiaries	(80)	(7)
Equity in earnings of subsidiaries, net of tax	280	254
Net income	200	247
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	200	247
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,940	2,907
Fiduciary investment income	6	7
Total revenue	2,946	2,914
Expenses
Compensation and benefits	1,731	1,699
Other general expenses	703	767
Total operating expenses	2,434	2,466
Operating income	512	448
Interest income	4	0
Interest expense	(18)	(16)
Intercompany interest income (expense)	(37)	(36)
Other income	1	10
Income before income taxes	462	406
Income tax (benefit) expense	108	106
Income (loss) before equity in earnings of subsidiaries	354	300
Equity in earnings of subsidiaries, net of tax	0	0
Net income	354	300
Less: Net income attributable to noncontrolling interests	11	11
Net income attributable to Aon shareholders	343	289
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
Other income	0	0
Income before income taxes	0	0
Income tax (benefit) expense	0	0
Income (loss) before equity in earnings of subsidiaries	0	0
Equity in earnings of subsidiaries, net of tax	(543)	(536)
Net income	(543)	(536)
Less: Net income attributable to noncontrolling interests	0	0
Net income attributable to Aon shareholders	$ (543)	$ (536)