Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Revenue
Commissions, fees and other	$ 2,913	$ 2,891	$ 5,854	$ 5,799
Fiduciary investment income	6	6	12	13
Total revenue	2,919	2,897	5,866	5,812
Expenses
Compensation and benefits	1,708	1,712	3,459	3,437
Other general expenses	766	803	1,493	1,583
Total operating expenses	2,474	2,515	4,952	5,020
Operating income	445	382	914	792
Interest income	2	2	4	3
Interest expense	(65)	(48)	(123)	(100)
Intercompany interest income (expense)	0	0	0	0
Other (expense) income	(2)	6	(1)	15
Income before income taxes	380	342	794	710
Income tax expense (benefit)	67	90	145	186
Income (loss) before equity in earnings of subsidiaries	313	252	649	524
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	313	252	649	524
Less: Net income attributable to noncontrolling interests	9	11	20	22
Net income attributable to Aon shareholders	304	241	629	502
Aon plc [Member]
Revenue
Commissions, fees and other	0	1	0	2
Fiduciary investment income	0	0	0	0
Total revenue	0	1	0	2
Expenses
Compensation and benefits	34	4	45	15
Other general expenses	5	9	13	16
Total operating expenses	39	13	58	31
Operating income	(39)	(12)	(58)	(29)
Interest income	(2)	0	(4)	0
Interest expense	(15)	(4)	(25)	(6)
Intercompany interest income (expense)	111	(7)	222	(14)
Other (expense) income	(1)	0	(1)	0
Income before income taxes	54	(23)	134	(49)
Income tax expense (benefit)	7	(3)	28	(9)
Income (loss) before equity in earnings of subsidiaries	47	(20)	106	(40)
Equity in earnings of subsidiaries, net of tax	257	261	523	542
Net income	304	241	629	502
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	304	241	629	502
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	6	6	15	21
Other general expenses	7	16	22	23
Total operating expenses	13	22	37	44
Operating income	(13)	(22)	(37)	(44)
Interest income	0	1	1	1
Interest expense	(32)	(33)	(63)	(67)
Intercompany interest income (expense)	(73)	43	(147)	86
Other (expense) income	8	(2)	8	(3)
Income before income taxes	(110)	(13)	(238)	(27)
Income tax expense (benefit)	(42)	(6)	(92)	(11)
Income (loss) before equity in earnings of subsidiaries	(68)	(7)	(146)	(16)
Equity in earnings of subsidiaries, net of tax	267	234	549	489
Net income	199	227	403	473
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	199	227	403	473
Other Non-Guarantor Subsidiaries [Member]
Revenue
Commissions, fees and other	2,913	2,890	5,854	5,797
Fiduciary investment income	6	6	12	13
Total revenue	2,919	2,896	5,866	5,810
Expenses
Compensation and benefits	1,668	1,702	3,399	3,401
Other general expenses	754	778	1,458	1,544
Total operating expenses	2,422	2,480	4,857	4,945
Operating income	497	416	1,009	865
Interest income	4	1	7	2
Interest expense	(18)	(11)	(35)	(27)
Intercompany interest income (expense)	(38)	(36)	(75)	(72)
Other (expense) income	(9)	8	(8)	18
Income before income taxes	436	378	898	786
Income tax expense (benefit)	102	99	209	206
Income (loss) before equity in earnings of subsidiaries	334	279	689	580
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income	334	279	689	580
Less: Net income attributable to noncontrolling interests	9	11	20	22
Net income attributable to Aon shareholders	325	268	669	558
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	0	0	0	0
Interest expense	0	0	0	0
Intercompany interest income (expense)	0	0	0	0
Other (expense) income	0	0	0	0
Income before income taxes	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Income (loss) before equity in earnings of subsidiaries	0	0	0	0
Equity in earnings of subsidiaries, net of tax	(524)	(495)	(1,072)	(1,031)
Net income	(524)	(495)	(1,072)	(1,031)
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income attributable to Aon shareholders	$ (524)	$ (495)	$ (1,072)	$ (1,031)