Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ (11)	$ 54
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	42	18
Purchases of long-term investments	(10)	(3)
Net sales of short-term investments - non-fiduciary	183	(16)
Acquisition of businesses, net of cash acquired	(5)	(2)
Proceeds from sale of businesses	1	1
Capital expenditures	(55)	(60)
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	156	(62)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(600)	(300)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	26	36
Issuance of debt	1,195	1,150
Repayment of debt	(829)	(715)
Cash dividends to shareholders	(53)	(50)
Purchase of shares from noncontrolling interests	1	(1)
Dividends paid to noncontrolling interests	0
CASH (USED FOR) PROVIDED BY FINANCING ACTIVITIES	(260)	120
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(24)	5
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(139)	117
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	477	291
CASH AND CASH EQUIVALENTS AT END OF PERIOD	338	408
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	89	5
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 PROVIDED BY (USED FOR) INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(600)	(300)
Advances from (to) affiliates	313	96
Issuance of shares for employee benefit plans	26	36
Issuance of debt	293	87
Repayment of debt	(68)	0
Cash dividends to shareholders	(53)	(50)
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0
CASH (USED FOR) PROVIDED BY FINANCING ACTIVITIES	(89)	(131)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	0	(126)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	0	131
CASH AND CASH EQUIVALENTS AT END OF PERIOD	0	5
Aon Corporation [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(123)	(23)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	37	1
Purchases of long-term investments	(10)	(3)
Net sales of short-term investments - non-fiduciary	62	(39)
Acquisition of businesses, net of cash acquired	0	0
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	89	(41)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(206)	(206)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	902	833
Repayment of debt	(755)	(511)
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0
CASH (USED FOR) PROVIDED BY FINANCING ACTIVITIES	(59)	116
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(93)	52
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	247	199
CASH AND CASH EQUIVALENTS AT END OF PERIOD	154	251
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	23	72
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	5	17
Purchases of long-term investments	0	0
Net sales of short-term investments - non-fiduciary	121	23
Acquisition of businesses, net of cash acquired	(5)	(2)
Proceeds from sale of businesses	1	1
Capital expenditures	(55)	(60)
CASH PROVIDED BY (USED FOR) INVESTING ACTIVITIES	67	(21)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	65	71
Issuance of shares for employee benefit plans	0	0
Issuance of debt	0	230
Repayment of debt	(6)	(204)
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	1	(1)
Dividends paid to noncontrolling interests	0
CASH (USED FOR) PROVIDED BY FINANCING ACTIVITIES	60	96
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(24)	5
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	126	152
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	1,246	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD	1,372	152
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 PROVIDED BY (USED FOR) INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(172)	39
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
CASH (USED FOR) PROVIDED BY FINANCING ACTIVITIES	(172)	39
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(172)	39
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(1,016)	(39)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ (1,188)	$ 0