Claims, Lawsuits and Other Contingencies	12 Months Ended
Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Claims, Lawsuits, and Other Contingencies	Claims, Lawsuits, and Other Contingencies Legal Aon and its subsidiaries are subject to numerous claims, tax assessments, lawsuits and proceedings that arise in the ordinary course of business, which include E&O claims. The damages claimed in these matters are or may be substantial, including, in many instances, claims for punitive, treble or extraordinary damages. While Aon maintains meaningful E&O insurance and other insurance programs to provide protection against certain losses that arise in such matters, Aon has exhausted or materially depleted its coverage under some of the policies that protect the Company and, consequently, is self-insured or materially self-insured for some claims. Accruals for these exposures, and related insurance receivables, when applicable, are included in the Consolidated Statements of Financial Position and have been recognized in Other general expenses in the Consolidated Statements of Income to the extent that losses are deemed probable and are reasonably estimable. These amounts are adjusted from time to time as developments warrant. Matters that are not probable and reasonably estimable are not accrued for in the financial statements. The Company has included in the current matters described below certain matters in which (1) loss is probable, (2) loss is reasonably possible; that is, more than remote but not probable, or (3) there exists the reasonable possibility of loss greater than the accrued amount. In addition, the Company may from time to time disclose matters for which the probability of loss could be remote but the claim amounts associated with such matters are potentially significant. The reasonably possible range of loss for the matters described below, in excess of amounts that are deemed probable and estimable and therefore already accrued, is estimated to be between $0 and $0.2 billion, exclusive of any insurance coverage. These estimates are based on currently available information. As available information changes, the matters for which Aon is able to estimate may change, and the estimates themselves may change. In addition, many estimates involve significant judgment and uncertainty. For example, at the time of making an estimate, Aon may only have limited information about the facts underlying the claim, and predictions and assumptions about future court rulings and outcomes may prove to be inaccurate. Although management at present believes that the ultimate outcome of all matters described below, individually or in the aggregate, will not have a material adverse effect on the consolidated financial position of Aon, legal proceedings are subject to inherent uncertainties and unfavorable rulings or other events. Unfavorable resolutions could include substantial monetary or punitive damages imposed on Aon or its subsidiaries. If unfavorable outcomes of these matters were to occur, future results of operations or cash flows for any particular quarterly or annual period could be materially adversely affected. Current Matters A pensions consulting and administration subsidiary of Aon provided advisory services to the Trustees of the Gleeds pension fund in the United Kingdom and, on occasion, to the relevant employer of the fund. In April 2014, the High Court, Chancery Division, London found that certain governing documents of the fund that sought to alter the fund’s benefit structure and that had been drafted by Aon were procedurally defective and therefore invalid. No lawsuit naming Aon as a party was filed, although a tolling agreement was entered. The High Court decision says that the additional liabilities in the pension fund resulting from the alleged defect in governing documents amount to approximately £45 million ($57 million at December 31, 2018 exchange rates). In December 2014, the Court of Appeal granted the employer leave to appeal the High Court decision. At a hearing in October 2016, the Court of Appeal approved a settlement of the pending litigation. On October 31, 2016, the fund’s trustees and employer sued Aon in the High Court, Chancery Division, London, alleging negligence and breach of duty in relation to the governing documents. The proceedings were served on Aon on December 20, 2016. The claimants seek damages of approximately £70 million ($88 million at December 31, 2018 exchange rates). In February 2018, the claimants instructed new lawyers and in May 2018 added their previous lawyers as defendants to the Aon lawsuit. The claimants allege that the previous lawyers were responsible for some of the losses sought from Aon because the lawyers gave negligent legal advice during the High Court and Court of Appeal proceedings. The trial of this matter has been set for November 2019. Aon believes that it has meritorious defenses and intends to vigorously defend itself against this claim. On June 29, 2015, Lyttelton Port Company Limited (“LPC”) sued Aon New Zealand in the Christchurch Registry of the High Court of New Zealand. LPC alleges, among other things, that Aon was negligent and in breach of contract in arranging LPC’s property insurance program for the period covering June 30, 2010, to June 30, 2011. LPC contends that acts and omissions by Aon caused LPC to recover less than it otherwise would have from insurers for losses suffered in the 2010 and 2011 Canterbury earthquakes. LPC claims damages of approximately NZD $184 million ($124 million at December 31, 2018 exchange rates) plus interest and costs. Aon believes that it has meritorious defenses and intends to vigorously defend itself against these claims. On October 3, 2017, Christchurch City Council (“CCC”) invoked arbitration to pursue a claim that it asserts against Aon New Zealand. Aon provided insurance broking services to CCC in relation to CCC’s 2010-2011 material damage and business interruption program. In December 2015, CCC settled its property and business interruption claim for its losses arising from the 2010-2011 Canterbury earthquakes against the underwriter of its material damage and business interruption program and the reinsurers of that underwriter. CCC contends that acts and omissions by Aon caused CCC to recover less in that settlement than it otherwise would have. CCC claims damages of approximately NZD $528 million ($355 million at December 31, 2018 exchange rates) plus interest and costs. Aon believes that it has meritorious defenses and intends to vigorously defend itself against these claims. A retail insurance brokerage subsidiary of Aon was sued on September 6, 2018 in the United States District Court for the Southern District of New York by a client, Pilkington North America, Inc., that sustained damage from a tornado to its Ottawa, Illinois property. The lawsuit seeks between $45 million and $85 million in property and business interruption damages from either its insurer or Aon. The insurer contends that insurance proceeds were limited to $15 million in coverage by a windstorm sub-limit purportedly contained in the policy procured by Aon for Pilkington. The insurer therefore has tendered $15 million to Pilkington and denied coverage for the remainder of the loss. Pilkington sued the insurer and Aon seeking full coverage for the loss from the insurer or, in the alternative, seeking the same damages against Aon on various theories of professional liability if the court finds that the $15 million sub-limit applies to the claim. Aon believes it has meritorious defenses and intends to vigorously defend itself against these claims. In April 2017, the FCA announced an investigation relating to suspected competition law breaches in the aviation and aerospace broking industry, which, for Aon in 2016, represented less than $100 million in global revenue. The European Commission has now assumed jurisdiction over the investigation in place of the FCA. Other antitrust agencies outside the European Union are also conducting formal or informal investigations regarding these matters. Aon intends to work diligently with all antitrust agencies concerned to ensure they can carry out their work as efficiently as possible. At this time, in light of the uncertainties and many variables involved, Aon cannot estimate the ultimate impact on our company from these investigations or any related private litigation, nor any damages, penalties, or fines related to them. There can be no assurance that the ultimate resolution of these matters will not have a material adverse effect on the Company’s consolidated financial position, results of operations, or liquidity. Settled/Closed Matters A retail insurance brokerage subsidiary of Aon was sued on September 14, 2010 in the Chancery Court for Davidson County, Tennessee Twentieth Judicial District, at Nashville by a client, Opry Mills Mall Limited Partnership (“Opry Mills”), that sustained flood damage to its property in May 2010. The lawsuit sought $200 million in coverage from numerous insurers with whom this Aon subsidiary placed the client’s property insurance coverage. The insurers contended that only $50 million in coverage (which had already been paid) was available for the loss because the flood event occurred on property in a high hazard flood zone. Opry Mills sought full coverage from the insurers for the loss and sued this Aon subsidiary in the alternative for the same $150 million difference on various theories of professional liability if the court determined there was not full coverage. In addition, Opry Mills sought prejudgment interest, attorneys’ fees and enhanced damages which could have substantially increased Aon’s exposure. In March 2015, the trial court granted partial summary judgment in favor of plaintiffs and against the insurers, holding generally that the plaintiffs are entitled to $200 million in coverage under the language of the policies. In August 2015, a jury returned a verdict in favor of Opry Mills and against the insurers in the amount of $204 million. On January 26, 2018, the Tennessee Court of Appeals reversed and remanded, reversing summary judgment in favor of plaintiffs and concluding that coverage is limited to $50 million. In December 2018, the parties reach an agreement to settle this case, and the settlement is now concluded. The terms of this settlement did not have a significant impact on Aon’s results of operations or financial condition. Guarantees and Indemnifications The Company provides a variety of guarantees and indemnifications to its customers and others. The maximum potential amount of future payments represents the notional amounts that could become payable under the guarantees and indemnifications if there were a total default by the guaranteed parties, without consideration of possible recoveries under recourse provisions or other methods. These amounts may bear no relationship to the expected future payments, if any, for these guarantees and indemnifications. Any anticipated amounts payable are included in the Company’s Consolidated Financial Statements, and are recorded at fair value. The Company expects that, as prudent business interests dictate, additional guarantees and indemnifications may be issued from time to time. Redomestication In connection with the Redomestication, the Company on April 2, 2012 entered into various agreements pursuant to which it agreed to guarantee the obligations of its subsidiaries arising under issued and outstanding debt securities. Those agreements included the (1) Amended and Restated Indenture, dated as of April 2, 2012, among Aon Corporation, Aon plc, and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Trustee”) (amending and restating the Indenture, dated as of September 10, 2010, between Aon Corporation and the Trustee), (2) Amended and Restated Indenture, dated as of April 2, 2012, among Aon Corporation, Aon plc and the Trustee (amending and restating the Indenture, dated as of December 16, 2002, between Aon Corporation and the Trustee), and (3) Amended and Restated Indenture, dated as of April 2, 2012, among Aon Corporation, Aon plc and the Trustee (amending and restating the Indenture, dated as of January 13, 1997, as supplemented by the First Supplemental Indenture, dated as of January 13, 1997). Sale of the Divested Business In connection with the sale of the Divested Business, the Company guaranteed future operating lease commitments related to certain facilities assumed by the Buyer. The Company is obligated to perform under the guarantees if the Divested Business defaults on such leases at any time during the remainder of the lease agreements, which expire on various dates through 2025. As of December 31, 2018, the undiscounted maximum potential future payments under the lease guarantee is $85 million, with an estimated fair value of $17 million. No cash payments were made in connection to the lease commitments during 2018. Additionally, the Company is subject to performance guarantee requirements under certain client arrangements that were assumed by the Buyer. Should the Divested Business fail to perform as required by the terms of the arrangements, the Company would be required to fulfill the remaining contract terms, which expire on various dates through 2023. As of December 31, 2018, the undiscounted maximum potential future payments under the performance guarantees were $188 million, with an estimated fair value of $1 million. No cash payments were made in connection to the lease commitments during 2018. Letters of Credit Aon has entered into a number of arrangements whereby the Company’s performance on certain obligations is guaranteed by a third party through the issuance of a letter of credit (“LOCs”). The Company had total LOCs outstanding of approximately $83 million at December 31, 2018, compared to $96 million at December 31, 2017. These letters of credit cover the beneficiaries related to certain of Aon’s U.S. and Canadian non-qualified pension plan schemes and secure deductible retentions for Aon’s own workers compensation program. The Company has also obtained LOCs to cover contingent payments for taxes and other business obligations to third parties, and other guarantees for miscellaneous purposes at its international subsidiaries. Premium Payments The Company has certain contractual contingent guarantees for premium payments owed by clients to certain insurance companies. The maximum exposure with respect to such contractual contingent guarantees was approximately $103 million at December 31, 2018, compared to $95 million at December 31, 2017.

Claims, Lawsuits, and Other Contingencies

Legal

Aon and its subsidiaries are subject to numerous claims, tax assessments, lawsuits and proceedings that arise in the ordinary course of business, which include E&O claims. The damages claimed in these matters are or may be substantial, including, in many instances, claims for punitive, treble or extraordinary damages. While Aon maintains meaningful E&O insurance and other insurance programs to provide protection against certain losses that arise in such matters, Aon has exhausted or materially depleted its coverage under some of the policies that protect the Company and, consequently, is self-insured or materially self-insured for some claims. Accruals for these exposures, and related insurance receivables, when applicable, are included in the Consolidated Statements of Financial Position and have been recognized in Other general expenses in the Consolidated Statements of Income to the extent that losses are deemed probable and are reasonably estimable. These amounts are adjusted from time to time as developments warrant. Matters that are not probable and reasonably estimable are not accrued for in the financial statements.

The Company has included in the current matters described below certain matters in which (1) loss is probable, (2) loss is reasonably possible; that is, more than remote but not probable, or (3) there exists the reasonable possibility of loss greater than the accrued amount. In addition, the Company may from time to time disclose matters for which the probability of loss could be remote but the claim amounts associated with such matters are potentially significant. The reasonably possible range of loss for the matters described below, in excess of amounts that are deemed probable and estimable and therefore already accrued, is estimated to be between $0 and $0.2 billion, exclusive of any insurance coverage. These estimates are based on currently available information. As available information changes, the matters for which Aon is able to estimate may change, and the estimates themselves may change. In addition, many estimates involve significant judgment and uncertainty. For example, at the time of making an estimate, Aon may only have limited information about the facts underlying the claim, and predictions and assumptions about future court rulings and outcomes may prove to be inaccurate. Although management at present believes that the ultimate outcome of all matters described below, individually or in the aggregate, will not have a material adverse effect on the consolidated financial position of Aon, legal proceedings are subject to inherent uncertainties and unfavorable rulings or other events. Unfavorable resolutions could include substantial monetary or punitive damages imposed on Aon or its subsidiaries. If unfavorable outcomes of these matters were to occur, future results of operations or cash flows for any particular quarterly or annual period could be materially adversely affected.

Current Matters

A pensions consulting and administration subsidiary of Aon provided advisory services to the Trustees of the Gleeds pension fund in the United Kingdom and, on occasion, to the relevant employer of the fund. In April 2014, the High Court, Chancery Division, London found that certain governing documents of the fund that sought to alter the fund’s benefit structure and that had been drafted by Aon were procedurally defective and therefore invalid. No lawsuit naming Aon as a party was filed, although a tolling agreement was entered. The High Court decision says that the additional liabilities in the pension fund resulting from the alleged defect in governing documents amount to approximately £45 million ($57 million at December 31, 2018 exchange rates). In December 2014, the Court of Appeal granted the employer leave to appeal the High Court decision. At a hearing in October 2016, the Court of Appeal approved a settlement of the pending litigation. On October 31, 2016, the fund’s trustees and employer sued Aon in the High Court, Chancery Division, London, alleging negligence and breach of duty in relation to the governing documents. The proceedings were served on Aon on December 20, 2016. The claimants seek damages of approximately £70 million ($88 million at December 31, 2018 exchange rates). In February 2018, the claimants instructed new lawyers and in May 2018 added their previous lawyers as defendants to the Aon lawsuit. The claimants allege that the previous lawyers were responsible for some of the losses sought from Aon because the lawyers gave negligent legal advice during the High Court and Court of Appeal proceedings. The trial of this matter has been set for November 2019. Aon believes that it has meritorious defenses and intends to vigorously defend itself against this claim.

On June 29, 2015, Lyttelton Port Company Limited (“LPC”) sued Aon New Zealand in the Christchurch Registry of the High Court of New Zealand. LPC alleges, among other things, that Aon was negligent and in breach of contract in arranging LPC’s property insurance program for the period covering June 30, 2010, to June 30, 2011. LPC contends that acts and omissions by Aon caused LPC to recover less than it otherwise would have from insurers for losses suffered in the 2010 and 2011 Canterbury earthquakes. LPC claims damages of approximately NZD $184 million ($124 million at December 31, 2018 exchange rates) plus interest and costs. Aon believes that it has meritorious defenses and intends to vigorously defend itself against these claims.

On October 3, 2017, Christchurch City Council (“CCC”) invoked arbitration to pursue a claim that it asserts against Aon New Zealand. Aon provided insurance broking services to CCC in relation to CCC’s 2010-2011 material damage and business interruption program. In December 2015, CCC settled its property and business interruption claim for its losses arising from the 2010-2011 Canterbury earthquakes against the underwriter of its material damage and business interruption program and the reinsurers of that underwriter. CCC contends that acts and omissions by Aon caused CCC to recover less in that settlement than it otherwise would have. CCC claims damages of approximately NZD $528 million ($355 million at December 31, 2018 exchange rates) plus interest and costs. Aon believes that it has meritorious defenses and intends to vigorously defend itself against these claims.

A retail insurance brokerage subsidiary of Aon was sued on September 6, 2018 in the United States District Court for the Southern District of New York by a client, Pilkington North America, Inc., that sustained damage from a tornado to its Ottawa, Illinois property. The lawsuit seeks between $45 million and $85 million in property and business interruption damages from either its insurer or Aon. The insurer contends that insurance proceeds were limited to $15 million in coverage by a windstorm sub-limit purportedly contained in the policy procured by Aon for Pilkington. The insurer therefore has tendered $15 million to Pilkington and denied coverage for the remainder of the loss. Pilkington sued the insurer and Aon seeking full coverage for the loss from the insurer or, in the alternative, seeking the same damages against Aon on various theories of professional liability if the court finds that the $15 million sub-limit applies to the claim. Aon believes it has meritorious defenses and intends to vigorously defend itself against these claims.

In April 2017, the FCA announced an investigation relating to suspected competition law breaches in the aviation and aerospace broking industry, which, for Aon in 2016, represented less than $100 million in global revenue. The European Commission has now assumed jurisdiction over the investigation in place of the FCA. Other antitrust agencies outside the European Union are also conducting formal or informal investigations regarding these matters. Aon intends to work diligently with all antitrust agencies concerned to ensure they can carry out their work as efficiently as possible. At this time, in light of the uncertainties and many variables involved, Aon cannot estimate the ultimate impact on our company from these investigations or any related private litigation, nor any damages, penalties, or fines related to them. There can be no assurance that the ultimate resolution of these matters will not have a material adverse effect on the Company’s consolidated financial position, results of operations, or liquidity.

Settled/Closed Matters

A retail insurance brokerage subsidiary of Aon was sued on September 14, 2010 in the Chancery Court for Davidson County, Tennessee Twentieth Judicial District, at Nashville by a client, Opry Mills Mall Limited Partnership (“Opry Mills”), that sustained flood damage to its property in May 2010. The lawsuit sought $200 million in coverage from numerous insurers with whom this Aon subsidiary placed the client’s property insurance coverage. The insurers contended that only $50 million in coverage (which had already been paid) was available for the loss because the flood event occurred on property in a high hazard flood zone. Opry Mills sought full coverage from the insurers for the loss and sued this Aon subsidiary in the alternative for the same $150 million difference on various theories of professional liability if the court determined there was not full coverage. In addition, Opry Mills sought prejudgment interest, attorneys’ fees and enhanced damages which could have substantially increased Aon’s exposure. In March 2015, the trial court granted partial summary judgment in favor of plaintiffs and against the insurers, holding generally that the plaintiffs are entitled to $200 million in coverage under the language of the policies. In August 2015, a jury returned a verdict in favor of Opry Mills and against the insurers in the amount of $204 million. On January 26, 2018, the Tennessee Court of Appeals reversed and remanded, reversing summary judgment in favor of plaintiffs and concluding that coverage is limited to $50 million. In December 2018, the parties reach an agreement to settle this case, and the settlement is now concluded. The terms of this settlement did not have a significant impact on Aon’s results of operations or financial condition.

Guarantees and Indemnifications

The Company provides a variety of guarantees and indemnifications to its customers and others. The maximum potential amount of future payments represents the notional amounts that could become payable under the guarantees and indemnifications if there were a total default by the guaranteed parties, without consideration of possible recoveries under recourse provisions or other methods. These amounts may bear no relationship to the expected future payments, if any, for these guarantees and indemnifications. Any anticipated amounts payable are included in the Company’s Consolidated Financial Statements, and are recorded at fair value.

The Company expects that, as prudent business interests dictate, additional guarantees and indemnifications may be issued from time to time.

Redomestication

In connection with the Redomestication, the Company on April 2, 2012 entered into various agreements pursuant to which it agreed to guarantee the obligations of its subsidiaries arising under issued and outstanding debt securities. Those agreements included the (1) Amended and Restated Indenture, dated as of April 2, 2012, among Aon Corporation, Aon plc, and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Trustee”) (amending and restating the Indenture, dated as of September 10, 2010, between Aon Corporation and the Trustee), (2) Amended and Restated Indenture, dated as of April 2, 2012, among Aon Corporation, Aon plc and the Trustee (amending and restating the Indenture, dated as of December 16, 2002, between Aon Corporation and the Trustee), and (3) Amended and Restated Indenture, dated as of April 2, 2012, among Aon Corporation, Aon plc and the Trustee (amending and restating the Indenture, dated as of January 13, 1997, as supplemented by the First Supplemental Indenture, dated as of January 13, 1997).

Sale of the Divested Business

In connection with the sale of the Divested Business, the Company guaranteed future operating lease commitments related to certain facilities assumed by the Buyer. The Company is obligated to perform under the guarantees if the Divested Business defaults on such leases at any time during the remainder of the lease agreements, which expire on various dates through 2025. As of December 31, 2018, the undiscounted maximum potential future payments under the lease guarantee is $85 million, with an estimated fair value of $17 million. No cash payments were made in connection to the lease commitments during 2018.

Additionally, the Company is subject to performance guarantee requirements under certain client arrangements that were assumed by the Buyer. Should the Divested Business fail to perform as required by the terms of the arrangements, the Company would be required to fulfill the remaining contract terms, which expire on various dates through 2023. As of December 31, 2018, the undiscounted maximum potential future payments under the performance guarantees were $188 million, with an estimated fair value of $1 million. No cash payments were made in connection to the lease commitments during 2018.

Letters of Credit

Aon has entered into a number of arrangements whereby the Company’s performance on certain obligations is guaranteed by a third party through the issuance of a letter of credit (“LOCs”). The Company had total LOCs outstanding of approximately $83 million at December 31, 2018, compared to $96 million at December 31, 2017. These letters of credit cover the beneficiaries related to certain of Aon’s U.S. and Canadian non-qualified pension plan schemes and secure deductible retentions for Aon’s own workers compensation program. The Company has also obtained LOCs to cover contingent payments for taxes and other business obligations to third parties, and other guarantees for miscellaneous purposes at its international subsidiaries.

Premium Payments

The Company has certain contractual contingent guarantees for premium payments owed by clients to certain insurance companies. The maximum exposure with respect to such contractual contingent guarantees was approximately $103 million at December 31, 2018, compared to $95 million at December 31, 2017.