Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Cash provided by (used for) operating activities	$ 1,163	$ 975
Cash flows from investing activities
Proceeds from investments	33	30
Payments for investments	(94)	(65)
Net sales (purchases) of short-term investments - non-fiduciary	(7)	356
Acquisition of businesses, net of cash acquired	(39)	(50)
Sale of businesses, net of cash sold	43	(8)
Capital expenditures	(167)	(179)
Cash provided by (used for) investing activities	(231)	84
Cash flows from financing activities
Share repurchase	(1,507)	(1,272)
Advances from (to) affiliates	0	0
Issuance of shares for employee benefit plans	(133)	(139)
Issuance of debt	4,918	3,960
Repayment of debt	(3,858)	(3,498)
Cash dividends to shareholders	(307)	(285)
Noncontrolling interests and other financing activities	(81)	(21)
Cash used for financing activities	(968)	(1,255)
Effect of exchange rates on cash and cash equivalents	(18)	(76)
Net increase (decrease) in cash and cash equivalents	(54)	(272)
Cash and cash equivalents at beginning of period	656	756
Cash and cash equivalents at end of period	602	484
Aon plc
Cash flows from operating activities
Cash provided by (used for) operating activities	356	(143)
Cash flows from investing activities
Proceeds from investments		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
Cash provided by (used for) investing activities	0	(12)
Cash flows from financing activities
Share repurchase	(1,507)	(1,272)
Advances from (to) affiliates	1,277	1,292
Issuance of shares for employee benefit plans	(133)	(139)
Issuance of debt	2,099	1,258
Repayment of debt	(1,785)	(700)
Cash dividends to shareholders	(307)	(285)
Noncontrolling interests and other financing activities	0	0
Cash used for financing activities	(356)	154
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	(118)	479
Cash flows from investing activities
Proceeds from investments	17	16
Payments for investments	(46)	(36)
Net sales (purchases) of short-term investments - non-fiduciary	(7)	308
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	(36)	288
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	79	(1,564)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	2,816	2,701
Repayment of debt	(2,073)	(2,501)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	0	0
Cash used for financing activities	822	(1,364)
Effect of exchange rates on cash and cash equivalents	0	0
Net increase (decrease) in cash and cash equivalents	668	(597)
Cash and cash equivalents at beginning of period	862	2,524
Cash and cash equivalents at end of period	1,530	1,927
Other Non-Guarantor Subsidiaries
Cash flows from operating activities
Cash provided by (used for) operating activities	1,578	2,258
Cash flows from investing activities
Proceeds from investments	16	14
Payments for investments	(49)	(29)
Net sales (purchases) of short-term investments - non-fiduciary	0	48
Acquisition of businesses, net of cash acquired	(39)	(50)
Sale of businesses, net of cash sold	43	(8)
Capital expenditures	(167)	(179)
Cash provided by (used for) investing activities	(196)	(204)
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	(1,370)	(2,054)
Issuance of shares for employee benefit plans	0	0
Issuance of debt	3	1
Repayment of debt	0	(297)
Cash dividends to shareholders	0	0
Noncontrolling interests and other financing activities	(81)	(21)
Cash used for financing activities	(1,448)	(2,371)
Effect of exchange rates on cash and cash equivalents	(18)	(76)
Net increase (decrease) in cash and cash equivalents	(84)	(393)
Cash and cash equivalents at beginning of period	575	793
Cash and cash equivalents at end of period	491	400
Consolidation Adjustments
Cash flows from operating activities
Cash provided by (used for) operating activities	(653)	(1,619)
Cash flows from investing activities
Proceeds from investments		0
Payments for investments	1	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	1	12
Cash flows from financing activities
Share repurchase	0	0
Advances from (to) affiliates	14	2,326
Issuance of shares for employee benefit plans	0	0
Issuance of debt		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	14	2,326
Effect of exchange rates on cash and cash equivalents	0	0
Net increase (decrease) in cash and cash equivalents	(638)	719
Cash and cash equivalents at beginning of period	(781)	(2,562)
Cash and cash equivalents at end of period	$ (1,419)	$ (1,843)

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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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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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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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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