Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 387	$ 269
CASH FLOWS FROM INVESTING ACTIVITIES
Sales of long-term investments	23	51
Purchases of long-term investments	(6)	(7)
Net (purchases) sales of short-term investments - non-fiduciary	29	259
Acquisition of businesses, net of cash acquired	(23)	(82)
Proceeds from sale of businesses	1	1
Capital expenditures	(122)	(129)
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(98)	93
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(525)	(350)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	57	64
Issuance of debt	2,914	332
Repayment of debt	(2,607)	(305)
Cash dividends to shareholders	(105)	(102)
Purchase of shares from noncontrolling interests	0	1
Dividends paid to noncontrolling interests	(6)	(6)
CASH USED FOR FINANCING ACTIVITIES	(272)	(366)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(42)	18
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(25)	14
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	291	272
CASH AND CASH EQUIVALENTS AT END OF PERIOD	266	286
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(23)	(19)
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	0
Capital expenditures	0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(525)	(250)
Advances from (to) affiliates	126	325
Issuance of shares for employee benefit plans	57	15
Issuance of debt	639	0
Repayment of debt	(6)	0
Cash dividends to shareholders	(105)	(52)
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0	0
CASH USED FOR FINANCING ACTIVITIES	186	38
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	163	19
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	131	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD	294	19
Aon Corporation [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(89)	(99)
CASH FLOWS FROM INVESTING ACTIVITIES
Sales of long-term investments	2	0
Purchases of long-term investments	(6)	(7)
Net (purchases) sales of short-term investments - non-fiduciary	26	211
Acquisition of businesses, net of cash acquired	0	(55)
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	22	149
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	(100)
Advances from (to) affiliates	444	118
Issuance of shares for employee benefit plans	0	49
Issuance of debt	2,044	332
Repayment of debt	(2,362)	(298)
Cash dividends to shareholders	0	(50)
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0	0
CASH USED FOR FINANCING ACTIVITIES	126	51
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	59	101
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	199	(21)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	258	80
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	499	387
CASH FLOWS FROM INVESTING ACTIVITIES
Sales of long-term investments	21	51
Purchases of long-term investments	0	0
Net (purchases) sales of short-term investments - non-fiduciary	3	48
Acquisition of businesses, net of cash acquired	(23)	(27)
Proceeds from sale of businesses	1	1
Capital expenditures	(122)	(129)
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(120)	(56)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(323)	(443)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	231	0
Repayment of debt	(239)	(7)
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	0	1
Dividends paid to noncontrolling interests	(6)	(6)
CASH USED FOR FINANCING ACTIVITIES	(337)	(455)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(42)	18
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	0	(106)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	0	293
CASH AND CASH EQUIVALENTS AT END OF PERIOD	0	187
Consolidating Adjustments [Member]
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	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	(247)	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	(247)	0
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(247)	0
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(39)	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ (286)	$ 0