Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 136	$ (11)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	3	42
Purchases of long-term investments	(1)	(10)
Net sales of short-term investments - non-fiduciary	42	183
Acquisition of businesses, net of cash acquired	(21)	(5)
Proceeds from sale of businesses	41	1
Capital expenditures	(62)	(55)
CASH PROVIDED BY INVESTING ACTIVITIES	2	156
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(250)	(600)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	48	26
Issuance of debt	870	1,195
Repayment of debt	(686)	(829)
Cash dividends to shareholders	(71)	(53)
Purchase of shares from noncontrolling interests	(5)	1
Dividends paid to noncontrolling interests	(1)	0
Proceeds from sale-leaseback	0
CASH USED FOR FINANCING ACTIVITIES	(95)	(260)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(39)	(24)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	4	(139)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	374	477
CASH AND CASH EQUIVALENTS AT END OF PERIOD	378	338
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	2	89
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 INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(250)	(600)
Advances from (to) affiliates	157	313
Issuance of shares for employee benefit plans	48	26
Issuance of debt	114	293
Repayment of debt	0	(68)
Cash dividends to shareholders	(71)	(53)
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0
Proceeds from sale-leaseback	0
CASH USED FOR FINANCING ACTIVITIES	(2)	(89)
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	(188)	(123)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	0	37
Purchases of long-term investments	(1)	(10)
Net sales of short-term investments - non-fiduciary	(1)	62
Acquisition of businesses, net of cash acquired	0	0
Proceeds from sale of businesses	0	0
Capital expenditures	0	0
CASH PROVIDED BY INVESTING ACTIVITIES	(2)	89
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(385)	(206)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	756	902
Repayment of debt	(678)	(755)
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	0	0
Dividends paid to noncontrolling interests	0
Proceeds from sale-leaseback	0
CASH USED FOR FINANCING ACTIVITIES	(307)	(59)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(497)	(93)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	2,727	247
CASH AND CASH EQUIVALENTS AT END OF PERIOD	2,230	154
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	322	23
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sale of long-term investments	3	5
Purchases of long-term investments	0	0
Net sales of short-term investments - non-fiduciary	43	121
Acquisition of businesses, net of cash acquired	(21)	(5)
Proceeds from sale of businesses	41	1
Capital expenditures	(62)	(55)
CASH PROVIDED BY INVESTING ACTIVITIES	4	67
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(984)	65
Issuance of shares for employee benefit plans	0	0
Issuance of debt	0	0
Repayment of debt	(8)	(6)
Cash dividends to shareholders	0	0
Purchase of shares from noncontrolling interests	(5)	1
Dividends paid to noncontrolling interests	(1)
Proceeds from sale-leaseback	0
CASH USED FOR FINANCING ACTIVITIES	(998)	60
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(39)	(24)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(711)	126
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	1,361	1,246
CASH AND CASH EQUIVALENTS AT END OF PERIOD	650	1,372
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 INVESTING ACTIVITIES	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	1,212	(172)
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
Proceeds from sale-leaseback	0
CASH USED FOR FINANCING ACTIVITIES	1,212	(172)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	1,212	(172)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(3,714)	(1,016)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ (2,502)	$ (1,188)