Guarantee of Registered Securities (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CASH FLOWS FROM OPERATING ACTIVITIES
CASH (USED FOR) PROVIDED BY OPERATING ACTIVITIES	$ 1,419	$ 1,018	$ 783
CASH FLOWS FROM INVESTING ACTIVITIES
Sale of long-term investments	178	190	90
Purchase of long-term investments	(12)	(30)	(34)
Net sales of short-term investments - non-fiduciary	440	(8)	(337)
Acquisition of businesses, net of cash acquired	(160)	(97)	(2,078)
Capital expenditures	(269)	(241)	(180)
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	177	(186)	(2,539)
CASH FLOWS FROM FINANCING ACTIVITIES
Purchase of treasury shares	(1,125)	(828)	(250)
Issuance of shares for employee benefit plans	118	201	194
Issuance of debt	733	1,673	2,905
Repayment of debt	(1,077)	(1,688)	(816)
Cash dividends to shareholders	(204)	(200)	(175)
Purchase of shares from noncontrolling interests	(4)	(24)	(15)
Dividends paid to noncontrolling interests	(27)	(30)	(20)
CASH (USED FOR) PROVIDED BY FINANCING ACTIVITIES	(1,586)	(896)	1,823
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	9	(10)	62
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	19	(74)	129
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	272	346	217
CASH AND CASH EQUIVALENTS AT END OF YEAR	291	272	346
Aon plc
CASH FLOWS FROM OPERATING ACTIVITIES
CASH (USED FOR) PROVIDED BY OPERATING ACTIVITIES	(87)
CASH FLOWS FROM INVESTING ACTIVITIES
Acquisition of businesses, net of cash acquired	(50)
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	(50)
CASH FLOWS FROM FINANCING ACTIVITIES
Purchase of treasury shares	(1,025)
Advances from (to) affiliates	1,379
Issuance of shares for employee benefit plans	69
Cash dividends to shareholders	(155)
CASH (USED FOR) PROVIDED BY FINANCING ACTIVITIES	268
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	131
CASH AND CASH EQUIVALENTS AT END OF YEAR	131
Aon Corporation
CASH FLOWS FROM OPERATING ACTIVITIES
CASH (USED FOR) PROVIDED BY OPERATING ACTIVITIES	(105)	(201)	(99)
CASH FLOWS FROM INVESTING ACTIVITIES
Sale of long-term investments	79	133	54
Purchase of long-term investments	(12)	(20)	(24)
Net sales of short-term investments - non-fiduciary	232	115	(262)
Acquisition of businesses, net of cash acquired	(55)	(5)	(2,001)
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	244	223	(2,233)
CASH FLOWS FROM FINANCING ACTIVITIES
Purchase of treasury shares	(100)	(828)	(250)
Advances from (to) affiliates	510	768	163
Issuance of shares for employee benefit plans	49	201	194
Issuance of debt	732	1,290	2,592
Repayment of debt	(1,061)	(1,288)	(173)
Cash dividends to shareholders	(49)	(200)	(175)
CASH (USED FOR) PROVIDED BY FINANCING ACTIVITIES	81	(57)	2,351
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	220	(35)	19
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	(21)	14	(5)
CASH AND CASH EQUIVALENTS AT END OF YEAR	199	(21)	14
Other Non-Guarantor Subsidiaries
CASH FLOWS FROM OPERATING ACTIVITIES
CASH (USED FOR) PROVIDED BY OPERATING ACTIVITIES	1,611	1,219	882
CASH FLOWS FROM INVESTING ACTIVITIES
Sale of long-term investments	99	57	36
Purchase of long-term investments		(10)	(10)
Net sales of short-term investments - non-fiduciary	208	(123)	(75)
Acquisition of businesses, net of cash acquired	(55)	(92)	(77)
Capital expenditures	(269)	(241)	(180)
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	(17)	(409)	(306)
CASH FLOWS FROM FINANCING ACTIVITIES
Advances from (to) affiliates	(1,889)	(768)	(163)
Issuance of debt	1	383	313
Repayment of debt	(16)	(400)	(643)
Purchase of shares from noncontrolling interests	(4)	(24)	(15)
Dividends paid to noncontrolling interests	(27)	(30)	(20)
CASH (USED FOR) PROVIDED BY FINANCING ACTIVITIES	(1,935)	(839)	(528)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	9	(10)	62
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(332)	(39)	110
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	293	332	222
CASH AND CASH EQUIVALENTS AT END OF YEAR	$ (39)	$ 293	$ 332