Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	$ 2,326	$ 2,009	$ 1,812
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	43	220	52
Payments for investments	(64)	(266)	(20)
Net (purchases) sales of short-term investments - non-fiduciary	61	9	110
Acquisition of businesses, net of cash acquired	(879)	(16)	(479)
Proceeds from sale of businesses	107	205	48
Capital expenditures	(222)	(290)	(256)
CASH USED FOR INVESTING ACTIVITIES	(954)	(138)	(545)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,257)	(1,550)	(2,250)
Advances from (to) affiliates and other (1)	0	0	0
Issuance of shares for employee benefit plans	(129)	(30)	(105)
Issuance of debt	3,467	5,351	5,239
Repayment of debt	(2,945)	(5,098)	(3,918)
Cash dividends to shareholders	(345)	(323)	(273)
Noncontrolling interests and other financing activities	(77)	(39)	4
CASH USED FOR FINANCING ACTIVITIES	(1,286)	(1,689)	(1,303)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(39)	(172)	(67)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	47	10	(103)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	384	374	477
CASH AND CASH EQUIVALENTS AT END OF YEAR	431	384	374
Aon plc
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	2,705	695	769
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	0	0	0
Payments for investments	0	(13)	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	0	(13)	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(1,257)	(1,550)	(2,250)
Advances from (to) affiliates and other (1)	(2,008)	232	19
Issuance of shares for employee benefit plans	(129)	(29)	(105)
Issuance of debt	1,879	1,318	2,908
Repayment of debt	(845)	(330)	(1,068)
Cash dividends to shareholders	(345)	(323)	(273)
Noncontrolling interests and other financing activities		0	0
CASH USED FOR FINANCING ACTIVITIES	(2,705)	(682)	(769)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	0	0	0
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	0	0	0
CASH AND CASH EQUIVALENTS AT END OF YEAR	0	0	0
Aon Corporation
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(536)	464	(927)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	316	27	39
Payments for investments	(35)	(47)	(20)
Net (purchases) sales of short-term investments - non-fiduciary	70	(42)	(3)
Acquisition of businesses, net of cash acquired	(335)	0	0
Proceeds from sale of businesses	0	0	0
Capital expenditures	0	0	0
CASH USED FOR INVESTING ACTIVITIES	16	(62)	16
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0	0
Advances from (to) affiliates and other (1)	570	(326)	3,215
Issuance of shares for employee benefit plans	0	0	0
Issuance of debt	1,588	4,026	2,326
Repayment of debt	(2,088)	(4,746)	(2,150)
Cash dividends to shareholders	0	0	0
Noncontrolling interests and other financing activities	0	0	0
CASH USED FOR FINANCING ACTIVITIES	70	(1,046)	3,391
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(450)	(644)	2,480
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	2,083	2,727	247
CASH AND CASH EQUIVALENTS AT END OF YEAR	1,633	2,083	2,727
Other Non-Guarantor Subsidiaries
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	3,265	2,523	1,970
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	15	193	13
Payments for investments	(29)	(219)	0
Net (purchases) sales of short-term investments - non-fiduciary	(9)	51	113
Acquisition of businesses, net of cash acquired	(608)	(16)	(479)
Proceeds from sale of businesses	171	205	48
Capital expenditures	(222)	(290)	(256)
CASH USED FOR INVESTING ACTIVITIES	(682)	(76)	(561)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0	0
Advances from (to) affiliates and other (1)	(3,037)	(2,339)	(536)
Issuance of shares for employee benefit plans	0	(1)	0
Issuance of debt	0	7	5
Repayment of debt	(12)	(22)	(700)
Cash dividends to shareholders	0	0	0
Noncontrolling interests and other financing activities	(77)	(39)	4
CASH USED FOR FINANCING ACTIVITIES	(3,126)	(2,394)	(1,227)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(39)	(172)	(67)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(582)	(119)	115
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	1,242	1,361	1,246
CASH AND CASH EQUIVALENTS AT END OF YEAR	660	1,242	1,361
Consolidating Adjustments
CASH FLOWS FROM OPERATING ACTIVITIES
CASH PROVIDED BY (USED FOR) OPERATING ACTIVITIES	(3,108)	(1,673)	0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	(288)	0	0
Payments for investments	0	13	0
Net (purchases) sales of short-term investments - non-fiduciary	0	0	0
Acquisition of businesses, net of cash acquired	64	0	0
Proceeds from sale of businesses	(64)	0	0
Capital expenditures	0	0	0
CASH USED FOR INVESTING ACTIVITIES	(288)	13	0
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0	0
Advances from (to) affiliates and other (1)	4,475	2,433	(2,698)
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
Noncontrolling interests and other financing activities	0	0	0
CASH USED FOR FINANCING ACTIVITIES	4,475	2,433	(2,698)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0	0
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	1,079	773	(2,698)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	(2,941)	(3,714)	(1,016)
CASH AND CASH EQUIVALENTS AT END OF YEAR	$ (1,862)	$ (2,941)	$ (3,714)