Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 883	$ 984
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	51	81
Purchases of long-term investments	(19)	(13)
Net sales of short-term investments - non-fiduciary	301	(13)
Acquisition of businesses, net of cash acquired	(464)	(26)
Proceeds from sale of businesses	48	6
Capital expenditures	(179)	(174)
CASH USED FOR INVESTING ACTIVITIES	(262)	(139)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,750)	(1,025)
Advances (to) from affiliates	0	0
Issuance of shares for employee benefit plans	58	84
Issuance of debt	4,255	4,270
Repayment of debt	(3,073)	(3,870)
Cash dividends to shareholders	(201)	(159)
Purchase of shares from noncontrolling interests	1	(6)
Dividends paid to noncontrolling interests	(18)	(13)
Proceeds from sale-leaseback	25	0
CASH USED FOR FINANCING ACTIVITIES	(703)	(719)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(13)	(48)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(95)	78
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	477	291
CASH AND CASH EQUIVALENTS AT END OF PERIOD	382	369
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	309	(14)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	0	0
Purchases of long-term investments	0	0
Net 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	(1,750)	(1,025)
Advances (to) from affiliates	(253)	405
Issuance of shares for employee benefit plans	58	84
Issuance of debt	2,714	1,145
Repayment of debt	(877)	(449)
Cash dividends to shareholders	(201)	(159)
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0	0
Proceeds from sale-leaseback	0
CASH USED FOR FINANCING ACTIVITIES	(309)	1
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	0	(13)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	0	131
CASH AND CASH EQUIVALENTS AT END OF PERIOD	0	118
Aon Corporation [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(348)	(195)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	39	8
Purchases of long-term investments	(19)	(13)
Net sales of short-term investments - non-fiduciary	103	(35)
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	123	(40)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances (to) from affiliates	496	610
Issuance of shares for employee benefit plans	0	0
Issuance of debt	1,541	2,894
Repayment of debt	(1,491)	(3,177)
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0	0
Proceeds from sale-leaseback	0
CASH USED FOR FINANCING ACTIVITIES	546	327
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	321	92
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	247	199
CASH AND CASH EQUIVALENTS AT END OF PERIOD	568	291
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	922	1,193
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	12	73
Purchases of long-term investments	0	0
Net sales of short-term investments - non-fiduciary	198	22
Acquisition of businesses, net of cash acquired	(464)	(26)
Proceeds from sale of businesses	48	6
Capital expenditures	(179)	(174)
CASH USED FOR INVESTING ACTIVITIES	(385)	(99)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances (to) from affiliates	371	(1,014)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	0	231
Repayment of debt	(705)	(244)
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	1	(6)
Dividends paid to noncontrolling interests	(18)	(13)
Proceeds from sale-leaseback	25
CASH USED FOR FINANCING ACTIVITIES	(326)	(1,046)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(13)	(48)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	198	0
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	1,246	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD	1,444	0
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 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 (to) from affiliates	(614)	(1)
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
Proceeds from sale-leaseback	0
CASH USED FOR FINANCING ACTIVITIES	(614)	(1)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(614)	(1)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(1,016)	(39)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ (1,630)	$ (40)