Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015
CASH FLOWS FROM OPERATING ACTIVITIES
CASH (USED FOR) PROVIDED BY OPERATING ACTIVITIES	$ 764	$ 578
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	23	10
Purchases of investments	(29)	(1)
Net (purchases) sales of short-term investments - non-fiduciary	106	(97)
Acquisition of businesses, net of cash acquired	(183)	(23)
Proceeds from sale of businesses	103	52
Capital expenditures	(104)	(142)
CASH USED FOR INVESTING ACTIVITIES	(84)	(201)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	(750)	(550)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	(87)	(161)
Issuance of debt	2,056	2,445
Repayment of debt	(1,632)	(1,896)
Cash dividends to shareholders	(169)	(156)
Noncontrolling interests and other financing activities	(62)	(23)
CASH USED FOR FINANCING ACTIVITIES	(644)	(341)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	18	(43)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	54	(7)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	384	374
CASH AND CASH EQUIVALENTS AT END OF PERIOD	438	367
Aon plc [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH (USED FOR) PROVIDED BY OPERATING ACTIVITIES	237	93
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	0	0
Purchases of investments	0	0
Net (purchases) 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	(750)	(550)
Advances from (to) affiliates	(211)	172
Issuance of shares for employee benefit plans	(87)	(161)
Issuance of debt	1,239	712
Repayment of debt	(259)	(110)
Cash dividends to shareholders	(169)	(156)
Noncontrolling interests and other financing activities	0	0
CASH USED FOR FINANCING ACTIVITIES	(237)	(93)
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 (USED FOR) PROVIDED BY OPERATING ACTIVITIES	(625)	(368)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	14	0
Purchases of investments	(13)	(1)
Net (purchases) sales of short-term investments - non-fiduciary	92	41
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	93	40
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	348	(134)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	817	1,732
Repayment of debt	(1,367)	(1,776)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	0	0
CASH USED FOR FINANCING ACTIVITIES	(202)	(178)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(734)	(506)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	2,083	2,727
CASH AND CASH EQUIVALENTS AT END OF PERIOD	1,349	2,221
Other Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH (USED FOR) PROVIDED BY OPERATING ACTIVITIES	1,156	853
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	9	10
Purchases of investments	(16)	0
Net (purchases) sales of short-term investments - non-fiduciary	14	(138)
Acquisition of businesses, net of cash acquired	(183)	(23)
Proceeds from sale of businesses	103	52
Capital expenditures	(104)	(142)
CASH USED FOR INVESTING ACTIVITIES	(177)	(241)
CASH FLOWS FROM FINANCING ACTIVITIES
Share repurchase	0	0
Advances from (to) affiliates	(321)	(1,191)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	0	1
Repayment of debt	(6)	(10)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	(62)	(23)
CASH USED FOR FINANCING ACTIVITIES	(389)	(1,223)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	18	(43)
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	608	(654)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	1,242	1,361
CASH AND CASH EQUIVALENTS AT END OF PERIOD	1,850	707
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
CASH (USED FOR) PROVIDED BY OPERATING ACTIVITIES	(4)	0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from investments	0	0
Purchases of investments	0	0
Net (purchases) 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	184	1,153
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
Noncontrolling interests and other financing activities	0	0
CASH USED FOR FINANCING ACTIVITIES	184	1,153
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	0	0
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	180	1,153
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD	(2,941)	(3,714)
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$ (2,761)	$ (2,561)

The cash inflow or outflow from sales or purchases of marketable securities.

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Proceeds (Payment) Towards Issuance of Shares Under Incentive and Share-based Compensation Plans, Net of Payments Related to Tax Withholding for Share-based Compensation

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of increase (decrease) in cash and cash equivalents. Cash and cash equivalents are the amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Includes effect from exchange rate changes.

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Amount of increase (decrease) from the effect of exchange rate changes on cash and cash equivalent balances held in foreign currencies.

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The increase (decrease) in obligations owed to an entity that is controlling, under the control of, or within the same control group as the reporting entity by means of direct or indirect ownership.

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Amount of cash inflow (outflow) from financing activities, including discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

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Amount of cash inflow (outflow) from investing activities, including discontinued operations. Investing activity cash flows include making and collecting loans and acquiring and disposing of debt or equity instruments and property, plant, and equipment and other productive assets.

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Amount of cash inflow (outflow) from operating activities, including discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

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The cash outflow to reacquire common stock during the period.

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Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity.

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The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase.

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The cash outflow associated with other investments held by the entity for investment purposes not otherwise defined in the taxonomy.

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The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

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Amount of cash outflow to a noncontrolling interest. Includes, but not limited to, reduction of noncontrolling interest ownership. Excludes dividends paid to the noncontrolling interest.

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This element represents the cash inflow during the period from the sale of a component of the entity.

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The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

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The cash inflow associated with the sale and maturity (principal being due) of other investments, prepayment and call (request of early payment) of other investments not otherwise defined in the taxonomy.

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The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

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