Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 984	$ 867
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	81	171
Purchases of long-term investments	(13)	(9)
Net (purchases) sales of short-term investments - non-fiduciary	(13)	66
Acquisition of businesses, net of cash acquired	(26)	(97)
Proceeds from sale of businesses	6	2
Capital expenditures	(174)	(201)
CASH USED FOR INVESTING ACTIVITIES	(139)	(68)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,025)	(625)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	84	93
Issuance of debt	4,270	333
Repayment of debt	(3,870)	(427)
Cash dividends to shareholders	(159)	(153)
Purchase of shares from noncontrolling interests	(6)	0
Dividends paid to noncontrolling interests	(13)	(17)
CASH USED FOR FINANCING ACTIVITIES	(719)	(796)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(48)	15
NET INCREASE IN CASH AND CASH EQUIVALENTS	78	18
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	291	272
CASH AND CASH EQUIVALENTS AT END OF PERIOD	369	290
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(14)	(46)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	0	0
Purchases of long-term investments	0	0
Net (purchases) sales of short-term investments - non-fiduciary	0	(16)
Acquisition of businesses, net of cash acquired	0	0
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH USED FOR INVESTING ACTIVITIES	0	(16)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,025)	(525)
Advances from (to) affiliates	405	648
Issuance of shares for employee benefit plans	84	44
Issuance of debt	1,145	0
Repayment of debt	(449)	0
Cash dividends to shareholders	(159)	(104)
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0	0
CASH USED FOR FINANCING ACTIVITIES	1	63
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE IN CASH AND CASH EQUIVALENTS	(13)	1
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	131	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD	118	1
Aon Corporation [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(195)	(189)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	8	79
Purchases of long-term investments	(13)	(9)
Net (purchases) sales of short-term investments - non-fiduciary	(35)	61
Acquisition of businesses, net of cash acquired	0	(55)
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH USED FOR INVESTING ACTIVITIES	(40)	76
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	(100)
Advances from (to) affiliates	610	376
Issuance of shares for employee benefit plans	0	49
Issuance of debt	2,894	332
Repayment of debt	(3,177)	(416)
Cash dividends to shareholders	0	(49)
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0	0
CASH USED FOR FINANCING ACTIVITIES	327	192
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE IN CASH AND CASH EQUIVALENTS	92	79
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	199	(21)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	291	58
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	1,193	1,102
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	73	92
Purchases of long-term investments	0	0
Net (purchases) sales of short-term investments - non-fiduciary	22	21
Acquisition of businesses, net of cash acquired	(26)	(42)
Proceeds from sale of businesses	6	2
Capital expenditures	(174)	(201)
CASH USED FOR INVESTING ACTIVITIES	(99)	(128)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(1,014)	(1,024)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	231	1
Repayment of debt	(244)	(11)
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	(6)	0
Dividends paid to noncontrolling interests	(13)	(17)
CASH USED FOR FINANCING ACTIVITIES	(1,046)	(1,051)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(48)	15
NET INCREASE IN CASH AND CASH EQUIVALENTS	0	(62)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	0	293
CASH AND CASH EQUIVALENTS AT END OF PERIOD	0	231
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	0	0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale 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	0
Capital expenditures	0	0
CASH USED FOR INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(1)	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	0
Dividends paid to noncontrolling interests	0	0
CASH USED FOR FINANCING ACTIVITIES	(1)	0
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE IN CASH AND CASH EQUIVALENTS	(1)	0
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(39)	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ (40)	$ 0