Guarantee of Registered Securities (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 54	$ (15)
CASH FLOWS FROM INVESTING ACTIVITIES
Sales of long-term investments	18	36
Purchases of long-term investments	(3)	(3)
Net (purchases) sales of short-term investments - non-fiduciary	(16)	283
Acquisition of businesses, net of cash acquired	(2)	(23)
Proceeds from sale of businesses	1	0
Capital expenditures	(60)	(71)
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(62)	222
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(300)	(100)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	36	49
Issuance of debt	1,150	75
Repayment of debt	(715)	(140)
Cash dividends to shareholders	(50)	(49)
Purchase of shares from noncontrolling interests	(1)	0
Dividends paid to noncontrolling interests	0	(1)
CASH PROVIDED BY (USED FOR) FINANCING ACTIVITIES	120	(166)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	5	10
NET INCREASE IN CASH AND CASH EQUIVALENTS	117	51
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	291	272
CASH AND CASH EQUIVALENTS AT END OF PERIOD	408	323
Aon plc
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	5	0
CASH FLOWS FROM INVESTING ACTIVITIES
Sales of long-term investments	0	0
Purchases of long-term investments	0	0
Net (purchases) sales of short-term investments - non-fiduciary	0	0
Acquisition of businesses, net of cash acquired	0	0
Proceeds from sale of businesses	0
Capital expenditures	0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(300)	0
Advances from (to) affiliates	96	17
Issuance of shares for employee benefit plans	36	0
Issuance of debt	87	0
Repayment of debt	0	0
Cash dividends to shareholders	(50)	0
Purchase of shares from noncontrolling interests	0
Dividends paid to noncontrolling interests		0
CASH PROVIDED BY (USED FOR) FINANCING ACTIVITIES	(131)	17
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE IN CASH AND CASH EQUIVALENTS	(126)	17
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	131	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD	5	17
Aon Corporation
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(23)	(66)
CASH FLOWS FROM INVESTING ACTIVITIES
Sales of long-term investments	1	0
Purchases of long-term investments	(3)	(3)
Net (purchases) sales of short-term investments - non-fiduciary	(39)	236
Acquisition of businesses, net of cash acquired	0	0
Proceeds from sale of businesses	0
Capital expenditures	0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(41)	233
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	(100)
Advances from (to) affiliates	(206)	(14)
Issuance of shares for employee benefit plans	0	49
Issuance of debt	833	75
Repayment of debt	(511)	(137)
Cash dividends to shareholders	0	(49)
Purchase of shares from noncontrolling interests	0
Dividends paid to noncontrolling interests		0
CASH PROVIDED BY (USED FOR) FINANCING ACTIVITIES	116	(176)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE IN CASH AND CASH EQUIVALENTS	52	(9)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	199	(21)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	251	(30)
Other Non-Guarantor Subsidiaries
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	72	51
CASH FLOWS FROM INVESTING ACTIVITIES
Sales of long-term investments	17	36
Purchases of long-term investments	0	0
Net (purchases) sales of short-term investments - non-fiduciary	23	47
Acquisition of businesses, net of cash acquired	(2)	(23)
Proceeds from sale of businesses	1
Capital expenditures	(60)	(71)
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(21)	(11)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	110	(3)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	230	0
Repayment of debt	(204)	(3)
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	(1)
Dividends paid to noncontrolling interests		(1)
CASH PROVIDED BY (USED FOR) FINANCING ACTIVITIES	135	(7)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	5	10
NET INCREASE IN CASH AND CASH EQUIVALENTS	191	43
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(39)	293
CASH AND CASH EQUIVALENTS AT END OF PERIOD	152	336
Consolidating Adjustments
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	0	0
CASH FLOWS FROM INVESTING ACTIVITIES
Sales of long-term investments	0	0
Purchases of long-term investments	0	0
Net (purchases) sales of short-term investments - non-fiduciary	0	0
Acquisition of businesses, net of cash acquired	0	0
Proceeds from sale of businesses	0
Capital expenditures	0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	0	0
Issuance of debt	0	0
Repayment of debt	0	0
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	0
Dividends paid to noncontrolling interests		0
CASH PROVIDED BY (USED FOR) FINANCING ACTIVITIES	0	0
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE IN CASH AND CASH EQUIVALENTS	0	0
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	0	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ 0	$ 0