Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 1,075	$ 883
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	23	51
Purchases of long-term investments	(3)	(19)
Net sales of short-term investments - non-fiduciary	(9)	301
Acquisition of businesses, net of cash acquired	(26)	(464)
Proceeds from sale of businesses	54	48
Capital expenditures	(225)	(179)
CASH USED FOR INVESTING ACTIVITIES	(186)	(262)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,150)	(1,750)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	72	58
Issuance of debt	3,494	4,255
Repayment of debt	(2,860)	(3,073)
Cash dividends to shareholders	(240)	(201)
Purchase of shares from noncontrolling interests	(5)	1
Dividends paid to noncontrolling interests	(21)	(18)
Proceeds from sale-leaseback	0	25
CASH USED FOR FINANCING ACTIVITIES	(710)	(703)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(155)	(13)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	24	(95)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	374	477
CASH AND CASH EQUIVALENTS AT END OF PERIOD	398	382
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	31	309
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	(1,150)	(1,750)
Advances from (to) affiliates	630	(253)
Issuance of shares for employee benefit plans	72	58
Issuance of debt	768	2,714
Repayment of debt	(111)	(877)
Cash dividends to shareholders	(240)	(201)
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	(31)	(309)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	0	0
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	0	0
CASH AND CASH EQUIVALENTS AT END OF PERIOD	0	0
Aon Corporation [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(624)	(348)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	3	39
Purchases of long-term investments	(1)	(19)
Net sales of short-term investments - non-fiduciary	(51)	103
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	(49)	123
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	451	496
Issuance of shares for employee benefit plans	0	0
Issuance of debt	2,720	1,541
Repayment of debt	(2,735)	(1,491)
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	436	546
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(237)	321
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	2,727	247
CASH AND CASH EQUIVALENTS AT END OF PERIOD	2,490	568
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	1,668	922
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	20	12
Purchases of long-term investments	(2)	0
Net sales of short-term investments - non-fiduciary	42	198
Acquisition of businesses, net of cash acquired	(26)	(464)
Proceeds from sale of businesses	54	48
Capital expenditures	(225)	(179)
CASH USED FOR INVESTING ACTIVITIES	(137)	(385)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(1,616)	371
Issuance of shares for employee benefit plans	0	0
Issuance of debt	6	0
Repayment of debt	(14)	(705)
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	(5)	1
Dividends paid to noncontrolling interests	(21)	(18)
Proceeds from sale-leaseback		25
CASH USED FOR FINANCING ACTIVITIES	(1,650)	(326)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(155)	(13)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(274)	198
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	1,361	1,246
CASH AND CASH EQUIVALENTS AT END OF PERIOD	1,087	1,444
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 from (to) affiliates	535	(614)
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	535	(614)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	535	(614)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(3,714)	(1,016)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ (3,179)	$ (1,630)