Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 2,009	$ 1,812	$ 1,753
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	220	52	93
Payments for investments	(266)	(20)	(15)
Net (purchases) sales of short-term investments - non-fiduciary	9	110	(174)
Acquisition of businesses, net of cash acquired	(16)	(479)	(54)
Proceeds from sale of businesses	205	48	40
Capital expenditures	(290)	(256)	(229)
CASH USED FOR INVESTING ACTIVITIES	(138)	(545)	(339)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,550)	(2,250)	(1,102)
Advances from (to) affiliates and other (1)	0	0	0
Issuance of shares for employee benefit plans	(30)	(105)	(22)
Issuance of debt	5,351	5,239	4,906
Repayment of debt	(5,098)	(3,918)	(4,679)
Cash dividends to shareholders	(323)	(273)	(212)
Purchases of shares from noncontrolling interests	(39)	4	(27)
CASH USED FOR FINANCING ACTIVITIES	(1,689)	(1,303)	(1,136)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(172)	(67)	(92)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	10	(103)	186
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	374	477	291
CASH AND CASH EQUIVALENTS AT END OF YEAR	384	374	477
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	695	769	70
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	0	0	0
Payments for investments	(13)	0	0
Net (purchases) sales of short-term investments - non-fiduciary	0	0	0
Acquisition of businesses, net of cash acquired	0	0	0
Proceeds from sale of businesses	0	0	0
Capital expenditures	0	0	0
CASH USED FOR INVESTING ACTIVITIES	(13)	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,550)	(2,250)	(1,102)
Advances from (to) affiliates and other (1)	232	19	460
Issuance of shares for employee benefit plans	(29)	(105)	(22)
Issuance of debt	1,318	2,908	1,730
Repayment of debt	(330)	(1,068)	(1,055)
Cash dividends to shareholders	(323)	(273)	$ (212)
Purchases of shares from noncontrolling interests	0	0
CASH USED FOR FINANCING ACTIVITIES	(682)	(769)	$ (201)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	0	0	(131)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	0	0	131
CASH AND CASH EQUIVALENTS AT END OF YEAR	0	0	0
Aon Corporation [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	464	(927)	(441)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	27	39	8
Payments for investments	(47)	(20)	(15)
Net (purchases) sales of short-term investments - non-fiduciary	(42)	(3)	(74)
Acquisition of businesses, net of cash acquired	0	0	0
Proceeds from sale of businesses	0	0	7
Capital expenditures	0	0	0
CASH USED FOR INVESTING ACTIVITIES	(62)	16	(74)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0	0
Advances from (to) affiliates and other (1)	(326)	3,215	996
Issuance of shares for employee benefit plans	0	0	0
Issuance of debt	4,026	2,326	2,944
Repayment of debt	(4,746)	(2,150)	(3,377)
Cash dividends to shareholders	0	0	0
Purchases of shares from noncontrolling interests	0	0	0
CASH USED FOR FINANCING ACTIVITIES	(1,046)	3,391	563
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(644)	2,480	48
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	2,727	247	199
CASH AND CASH EQUIVALENTS AT END OF YEAR	2,083	2,727	247
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	2,523	1,970	2,124
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	193	13	85
Payments for investments	(219)	0	0
Net (purchases) sales of short-term investments - non-fiduciary	51	113	(100)
Acquisition of businesses, net of cash acquired	(16)	(479)	(54)
Proceeds from sale of businesses	205	48	33
Capital expenditures	(290)	(256)	(229)
CASH USED FOR INVESTING ACTIVITIES	(76)	(561)	(265)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0	0
Advances from (to) affiliates and other (1)	(2,339)	(536)	(479)
Issuance of shares for employee benefit plans	(1)	0	0
Issuance of debt	7	5	232
Repayment of debt	(22)	(700)	(247)
Cash dividends to shareholders	0	0	0
Purchases of shares from noncontrolling interests	(39)	4	(27)
CASH USED FOR FINANCING ACTIVITIES	(2,394)	(1,227)	(521)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(172)	(67)	(92)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(119)	115	1,246
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	1,361	1,246	0
CASH AND CASH EQUIVALENTS AT END OF YEAR	1,242	1,361	1,246
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(1,673)	0	0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	0	0	0
Payments for investments	13	0	0
Net (purchases) sales of short-term investments - non-fiduciary	0	0	0
Acquisition of businesses, net of cash acquired	0	0	0
Proceeds from sale of businesses	0	0	0
Capital expenditures	0	0	0
CASH USED FOR INVESTING ACTIVITIES	13	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0	0
Advances from (to) affiliates and other (1)	2,433	(2,698)	(977)
Issuance of shares for employee benefit plans	0	0	0
Issuance of debt	0	0	0
Repayment of debt	0	0	0
Cash dividends to shareholders	0	0	0
Purchases of shares from noncontrolling interests	0	0	0
CASH USED FOR FINANCING ACTIVITIES	2,433	(2,698)	(977)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	773	(2,698)	(977)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	(3,714)	(1,016)	(39)
CASH AND CASH EQUIVALENTS AT END OF YEAR	$ (2,941)	$ (3,714)	$ (1,016)