Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities
Cash provided by (used for) operating activities	$ 361	$ 413
Cash flows from investing activities
Proceeds from investments	14	23
Payments for investments	(60)	(36)
Net sales (purchases) of short-term investments - non-fiduciary	(62)	352
Acquisition of businesses, net of cash acquired	(15)	(50)
Sale of businesses, net of cash sold	7	1
Capital expenditures	(106)	(111)
Cash provided by (used for) investing activities	(222)	179
Cash flows from financing activities
Share repurchase	(1,155)	(971)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	(144)	(150)
Issuance of debt	3,559	2,552
Repayment of debt	(2,228)	(2,027)
Cash dividends to shareholders	(203)	(187)
Noncontrolling interests and other financing activities	(61)	(15)
Cash used for financing activities	(232)	(798)
Effect of exchange rates on cash and cash equivalents	18	(63)
Net increase (decrease) in cash and cash equivalents	(75)	(269)
Cash and cash equivalents at beginning of period	656	756
Cash and cash equivalents at end of period	581	487
Aon plc
Cash flows from operating activities
Cash provided by (used for) operating activities	366	(126)
Cash flows from investing activities
Proceeds from investments	0	0
Payments for investments	0	(12)
Net sales (purchases) of short-term investments - non-fiduciary	0	0
Acquisition of businesses, net of cash acquired	0	0
Sale of businesses, net of cash sold	0	0
Capital expenditures	0	0
Cash provided by (used for) investing activities	0	(12)
Cash flows from financing activities
Share repurchase	(1,155)	(971)
Advances from (to) affiliates	823	965
Issuance of shares for employee benefit plans	(144)	(150)
Issuance of debt	1,219	752
Repayment of debt	(906)	(272)
Cash dividends to shareholders	(203)	(187)
Noncontrolling interests and other financing activities	0	0
Cash used for financing activities	(366)	137
Effect of exchange rates on cash and cash equivalents	0	0
Net increase (decrease) in cash and cash equivalents	0	(1)
Cash and cash equivalents at beginning of period	0	1
Cash and cash equivalents at end of period	0	0
Aon Corporation
Cash flows from operating activities
Cash provided by (used for) operating activities	(110)	582
Cash flows from investing activities
Proceeds from investments	9	13
Payments for investments	(19)	(17)
Net sales (purchases) of short-term investments - non-fiduciary	(69)	296
Acquisition of businesses, net of cash acquired	0	0
Sale of businesses, net of cash sold	0	0
Capital expenditures	0	0
Cash provided by (used for) investing activities	(79)	292
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	(320)	(810)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	2,340	1,800
Repayment of debt	(1,322)	(1,461)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	0	0
Cash used for financing activities	698	(471)
Effect of exchange rates on cash and cash equivalents	0	0
Net increase (decrease) in cash and cash equivalents	509	403
Cash and cash equivalents at beginning of period	862	2,524
Cash and cash equivalents at end of period	1,371	2,927
Other Non-Guarantor Subsidiaries
Cash flows from operating activities
Cash provided by (used for) operating activities	690	759
Cash flows from investing activities
Proceeds from investments	5	10
Payments for investments	(41)	(19)
Net sales (purchases) of short-term investments - non-fiduciary	7	56
Acquisition of businesses, net of cash acquired	(15)	(50)
Sale of businesses, net of cash sold	7	1
Capital expenditures	(106)	(111)
Cash provided by (used for) investing activities	(143)	(113)
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	(542)	(395)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	0	0
Repayment of debt	0	(294)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	(61)	(15)
Cash used for financing activities	(603)	(704)
Effect of exchange rates on cash and cash equivalents	18	(63)
Net increase (decrease) in cash and cash equivalents	(38)	(121)
Cash and cash equivalents at beginning of period	575	793
Cash and cash equivalents at end of period	537	672
Consolidation Adjustments
Cash flows from operating activities
Cash provided by (used for) operating activities	(585)	(802)
Cash flows from investing activities
Proceeds from investments	0	0
Payments for investments	0	12
Net sales (purchases) of short-term investments - non-fiduciary	0	0
Acquisition of businesses, net of cash acquired	0	0
Sale of businesses, net of cash sold	0	0
Capital expenditures	0	0
Cash provided by (used for) investing activities	0	12
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	39	240
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	39	240
Effect of exchange rates on cash and cash equivalents	0	0
Net increase (decrease) in cash and cash equivalents	(546)	(550)
Cash and cash equivalents at beginning of period	(781)	(2,562)
Cash and cash equivalents at end of period	$ (1,327)	$ (3,112)

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 cash and cash equivalents, and cash and cash equivalents restricted to withdrawal or usage. Excludes amount for disposal group and discontinued operations. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, 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.

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Amount of increase (decrease) in cash, cash equivalents, and cash and cash equivalents restricted to withdrawal or usage; including effect from exchange rate change. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, 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.

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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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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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Amount of cash outflow to acquire investments classified as other.

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

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