Guarantee of Registered Securities Condensed Consolidating Statement of Cash Flows (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 1,633	$ 1,419	$ 1,018
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of long-term investments	93	178	190
Purchases of long-term investments	(15)	(12)	(30)
Net (purchases) sales of short-term investments - non-fiduciary	(174)	440	(8)
Acquisition of businesses, net of cash acquired	(54)	(162)	(106)
Proceeds from sale of businesses	40	2	9
Capital expenditures	(229)	(269)	(241)
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(339)	177	(186)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,102)	(1,125)	(828)
Advances from (to) affiliates	0	0	0
Issuance of shares for employee benefit plans	98	118	201
Issuance of debt	4,906	733	1,673
Repayment of debt	(4,679)	(1,077)	(1,688)
Cash dividends to shareholders	(212)	(204)	(200)
Purchases of shares from noncontrolling interests	(8)	(4)	(24)
Dividends paid to noncontrolling interests	(19)	(27)	(30)
CASH USED FOR FINANCING ACTIVITIES	(1,016)	(1,586)	(896)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(92)	9	(10)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	186	19	(74)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	291	272	346
CASH AND CASH EQUIVALENTS AT END OF YEAR	477	291	272
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(50)	(87)	0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of long-term investments	0	0	0
Purchases of long-term investments	0	0	0
Net (purchases) sales of short-term investments - non-fiduciary	0	0	0
Acquisition of businesses, net of cash acquired	0	(50)	0
Proceeds from sale of businesses	0	0	0
Capital expenditures	0	0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	0	(50)	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,102)	(1,025)	0
Advances from (to) affiliates	460	1,379	0
Issuance of shares for employee benefit plans	98	69	0
Issuance of debt	1,730	0	0
Repayment of debt	(1,055)	0	0
Cash dividends to shareholders	(212)	(155)	0
Purchases of shares from noncontrolling interests	0	0	0
Dividends paid to noncontrolling interests	0	0	0
CASH USED FOR FINANCING ACTIVITIES	(81)	268	0
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(131)	131	0
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	131	0	0
CASH AND CASH EQUIVALENTS AT END OF YEAR	0	131	0
Aon Corporation [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(441)	(105)	(201)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of long-term investments	8	79	133
Purchases of long-term investments	(15)	(12)	(20)
Net (purchases) sales of short-term investments - non-fiduciary	(74)	232	115
Acquisition of businesses, net of cash acquired	0	(55)	(10)
Proceeds from sale of businesses	7	0	5
Capital expenditures		0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(74)	244	223
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	(100)	(828)
Advances from (to) affiliates	996	489	789
Issuance of shares for employee benefit plans	0	49	201
Issuance of debt	2,944	732	1,290
Repayment of debt	(3,377)	(1,061)	(1,288)
Cash dividends to shareholders	0	(49)	(200)
Purchases of shares from noncontrolling interests	0	0	0
Dividends paid to noncontrolling interests	0	0	0
CASH USED FOR FINANCING ACTIVITIES	563	60	(36)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	48	199	(14)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	199	0	14
CASH AND CASH EQUIVALENTS AT END OF YEAR	247	199	0
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	2,124	1,611	1,219
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of long-term investments	85	99	57
Purchases of long-term investments	0	0	(10)
Net (purchases) sales of short-term investments - non-fiduciary	(100)	208	(123)
Acquisition of businesses, net of cash acquired	(54)	(57)	(96)
Proceeds from sale of businesses	33	2	4
Capital expenditures	(229)	(269)	(241)
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(265)	(17)	(409)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0	0
Advances from (to) affiliates	(479)	(1,850)	(768)
Issuance of shares for employee benefit plans	0	0	0
Issuance of debt	232	1	383
Repayment of debt	(247)	(16)	(400)
Cash dividends to shareholders	0	0	0
Purchases of shares from noncontrolling interests	(8)	(4)	(24)
Dividends paid to noncontrolling interests	(19)	(27)	(30)
CASH USED FOR FINANCING ACTIVITIES	(521)	(1,896)	(839)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(92)	9	(10)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	1,246	(293)	(39)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	0	293	332
CASH AND CASH EQUIVALENTS AT END OF YEAR	1,246	0	293
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	0	0	0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of long-term investments	0	0	0
Purchases of long-term investments	0	0	0
Net (purchases) sales of short-term investments - non-fiduciary	0	0	0
Acquisition of businesses, net of cash acquired	0	0	0
Proceeds from sale of businesses	0	0	0
Capital expenditures	0	0	0
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	0	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0	0
Advances from (to) affiliates	(977)	(18)	(21)
Issuance of shares for employee benefit plans	0	0	0
Issuance of debt	0	0	0
Repayment of debt	0	0	0
Cash dividends to shareholders	0	0	0
Purchases of shares from noncontrolling interests	0	0	0
Dividends paid to noncontrolling interests	0	0	0
CASH USED FOR FINANCING ACTIVITIES	(977)	(18)	(21)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(977)	(18)	(21)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	(39)	(21)	0
CASH AND CASH EQUIVALENTS AT END OF YEAR	$ (1,016)	$ (39)	$ (21)