Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 365	$ 333
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	10	48
Purchases of long-term investments	(1)	(14)
Net sales of short-term investments - non-fiduciary	(97)	217
Acquisition of businesses, net of cash acquired	(23)	(83)
Proceeds from sale of businesses	52	1
Capital expenditures	(142)	(115)
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(201)	54
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(550)	(1,250)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	52	40
Issuance of debt	2,445	3,324
Repayment of debt	(1,896)	(1,745)
Deposit with trustee	0	681
Cash dividends to shareholders	(156)	(128)
Purchase of shares from noncontrolling interests	(5)	1
Dividends paid to noncontrolling interests	(18)	(10)
CASH USED FOR FINANCING ACTIVITIES	(128)	(449)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(43)	3
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(7)	(59)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	374	477
CASH AND CASH EQUIVALENTS AT END OF PERIOD	367	418
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(120)	265
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) PROVIDED BY INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(550)	(1,250)
Advances from (to) affiliates	172	(522)
Issuance of shares for employee benefit plans	52	40
Issuance of debt	712	2,169
Repayment of debt	(110)	(574)
Deposit with trustee		0
Cash dividends to shareholders	(156)	(128)
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0	0
CASH USED FOR FINANCING ACTIVITIES	120	(265)
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	(368)	(215)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	0	37
Purchases of long-term investments	(1)	(14)
Net sales of short-term investments - non-fiduciary	41	27
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	40	50
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(134)	99
Issuance of shares for employee benefit plans	0	0
Issuance of debt	1,732	1,161
Repayment of debt	(1,776)	(1,161)
Deposit with trustee		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	(178)	99
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(506)	(66)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	2,727	247
CASH AND CASH EQUIVALENTS AT END OF PERIOD	2,221	181
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	853	283
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	10	11
Purchases of long-term investments	0	0
Net sales of short-term investments - non-fiduciary	(138)	190
Acquisition of businesses, net of cash acquired	(23)	(83)
Proceeds from sale of businesses	52	1
Capital expenditures	(142)	(115)
CASH (USED FOR) PROVIDED BY INVESTING ACTIVITIES	(241)	4
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(1,191)	586
Issuance of shares for employee benefit plans	0	0
Issuance of debt	1	(6)
Repayment of debt	(10)	(10)
Deposit with trustee		681
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	(5)	1
Dividends paid to noncontrolling interests	(18)	(10)
CASH USED FOR FINANCING ACTIVITIES	(1,223)	(120)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(43)	3
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(654)	170
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	1,361	1,246
CASH AND CASH EQUIVALENTS AT END OF PERIOD	707	1,416
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) PROVIDED BY INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	1,153	(163)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	0	0
Repayment of debt	0	0
Deposit with trustee		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	1,153	(163)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	1,153	(163)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(3,714)	(1,016)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ (2,561)	$ (1,179)