Commitments and Contingencies	6 Months Ended
Jun. 30, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and 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 frequently include errors and omissions ("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 Condensed Consolidated Statements of Financial Position and have been recognized in Other general expenses in the Condensed 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. We have included in the 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, we 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 $0.3 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 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 seeks $200 million in coverage from numerous insurers with whom this Aon subsidiary placed the client's property insurance coverage. The insurers contend that only $50 million in coverage (which has already been paid) is available for the loss because the flood event occurred on property in a high hazard flood zone. Opry Mills is seeking full coverage from the insurers for the loss and has sued this Aon subsidiary in the alternative for the same $150 million difference on various theories of professional liability if the court determines there is not full coverage. In addition, Opry Mills seeks prejudgment interest, attorneys' fees and enhanced damages which could substantially increase 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. Aon understands that the insurers intend to appeal both of these trial court decisions. Aon believes it has meritorious defenses and intends to vigorously defend itself against these claims. A pensions consulting and administration subsidiary of Hewitt before its acquisition by Aon provided advisory services to the Trustees of the Philips UK pension fund and the relevant employer of fund beneficiaries. On January 2, 2014, Philips Pension Trustees Limited and Philips Electronics UK Limited (together, "Philips") sued Aon in the High Court, Chancery Division, London alleging negligence and breach of duty. The proceedings assert Philips' right to claim damages related to Philips' use of a credit default swap hedging strategy pursuant to the supply of the advisory services, which is said to have resulted in substantial damages to Philips. Philips sought approximately £189 million ($259 million at June 30, 2016 exchange rates), plus interest and costs. In June 2015, the High Court ordered Philips to clarify several aspects of its claim. In its clarification, Philips increased the amount of its claim to £290 million ($397 million at June 30, 2016 exchange rates), plus interest and costs. Aon believes that it has meritorious defenses and intends to vigorously defend itself against these allegations. On June 1, 2007, the International Road Transport Union ("IRU") sued Aon in the Geneva Tribunal of First Instance in Switzerland. IRU alleges, among other things, that, between 1995 and 2004, a business acquired by Aon and, later, an Aon subsidiary (1) accepted commissions for certain insurance placements that violated a fee agreement entered between the parties and (2) negligently failed to ask certain insurance carriers to contribute to the IRU's risk management costs. IRU sought damages of approximately CHF 46 million ($47 million at June 30, 2016 exchange rates) and $3 million, plus legal fees and interest of approximately $30 million. On December 2, 2014, the Geneva Tribunal of First Instance entered a judgment that accepted some, and rejected other, of IRU's claims. The judgment awarded IRU CHF 16.8 million ($17 million at June 30, 2016 exchange rates) and $3.1 million, plus interest and adverse costs. The entire amount of the judgment, including interest through December 31, 2014, totaled CHF 27.9 million ($29 million at June 30, 2016 exchange rates) and $5 million. On January 26, 2015, in return for IRU agreeing not to appeal the bulk of its dismissed claims, the Aon subsidiary agreed not to appeal a part of the judgment and to pay IRU CHF 12.8 million ($13 million at June 30, 2016 exchange rates) and $4.7 million without Aon admitting liability. The Aon subsidiary appealed those aspects of the judgment it retained the right to appeal. IRU did not appeal. The Geneva Appellate Court affirmed the judgment of the Geneva Tribunal of First Instance. The Geneva Appellate Court’s judgment can be appealed to the Swiss Supreme Court. The Aon subsidiary's maximum liability on appeal is limited to CHF 8.7 million ($9 million at June 30, 2016 exchange rates) and $115,000 (plus interest and costs) beyond what the subsidiary has already paid. 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 has been filed, although a tolling agreement has been 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 ($62 million at June 30, 2016 exchange rates). In December 2014, the Court of Appeal granted the employer leave to appeal the High Court decision. The Court of Appeal hearing was set for October 2015, but has been postponed to permit the parties to discuss possible settlement. Aon believes that it has meritorious defenses and intends to vigorously defend itself against this potential 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 2010/2011 Canterbury Earthquakes. LPC claims damages of approximately NZD 184 million ($131 million at June 30, 2016 exchange rates) plus interest and costs. Aon believes that it has meritorious defenses and intends to vigorously defend itself against these claims. In addition, from time to time, Aon's clients may bring claims and take legal action pertaining to the performance of fiduciary responsibilities. Whether client claims and legal action related to the Company's performance of fiduciary responsibilities are founded or unfounded, if such claims and legal actions are resolved in a manner unfavorable to the Company, they may adversely affect Aon's financial results and materially impair the market perception of the Company and that of its products and services. Guarantees and Indemnifications In connection with the redomicile of Aon's headquarters (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), (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), and (4) First Supplemental Indenture, dated as of April 2, 2012, among Aon Finance N.S. 1, ULC, as issuer, Aon Corporation, as guarantor, Aon plc, as guarantor, and Computershare Trust Company of Canada, as trustee. 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 Condensed 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. Letters of Credit The Company had total letters of credit ("LOCs") outstanding of approximately $82 million at June 30, 2016, compared to $58 million at December 31, 2015. 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 the Company's own workers compensation program. The Company has also issued LOCs to cover contingent payments for taxes and other business obligations to third parties, and other guarantees for miscellaneous purposes at its international subsidiaries. Commitments The Company has provided commitments to fund certain limited partnerships in which it has an interest in the event that the general partners request funding. Some of these commitments have specific expiration dates and the maximum potential funding under these commitments was $12 million at June 30, 2016 and December 31, 2015. During the three and six months ended June 30, 2016, no significant commitments were funded. 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 $89 million at June 30, 2016 compared to $104 million at December 31, 2015.

Commitments and 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 frequently include errors and omissions ("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 Condensed Consolidated Statements of Financial Position and have been recognized in Other general expenses in the Condensed 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.

We have included in the 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, we 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 $0.3 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 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 seeks $200 million in coverage from numerous insurers with whom this Aon subsidiary placed the client's property insurance coverage. The insurers contend that only $50 million in coverage (which has already been paid) is available for the loss because the flood event occurred on property in a high hazard flood zone. Opry Mills is seeking full coverage from the insurers for the loss and has sued this Aon subsidiary in the alternative for the same $150 million difference on various theories of professional liability if the court determines there is not full coverage. In addition, Opry Mills seeks prejudgment interest, attorneys' fees and enhanced damages which could substantially increase 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. Aon understands that the insurers intend to appeal both of these trial court decisions. Aon believes it has meritorious defenses and intends to vigorously defend itself against these claims.

A pensions consulting and administration subsidiary of Hewitt before its acquisition by Aon provided advisory services to the Trustees of the Philips UK pension fund and the relevant employer of fund beneficiaries. On January 2, 2014, Philips Pension Trustees Limited and Philips Electronics UK Limited (together, "Philips") sued Aon in the High Court, Chancery Division, London alleging negligence and breach of duty. The proceedings assert Philips' right to claim damages related to Philips' use of a credit default swap hedging strategy pursuant to the supply of the advisory services, which is said to have resulted in substantial damages to Philips. Philips sought approximately £189 million ($259 million at June 30, 2016 exchange rates), plus interest and costs. In June 2015, the High Court ordered Philips to clarify several aspects of its claim. In its clarification, Philips increased the amount of its claim to £290 million ($397 million at June 30, 2016 exchange rates), plus interest and costs. Aon believes that it has meritorious defenses and intends to vigorously defend itself against these allegations.

On June 1, 2007, the International Road Transport Union ("IRU") sued Aon in the Geneva Tribunal of First Instance in Switzerland. IRU alleges, among other things, that, between 1995 and 2004, a business acquired by Aon and, later, an Aon subsidiary (1) accepted commissions for certain insurance placements that violated a fee agreement entered between the parties and (2) negligently failed to ask certain insurance carriers to contribute to the IRU's risk management costs. IRU sought damages of approximately CHF 46 million ($47 million at June 30, 2016 exchange rates) and $3 million, plus legal fees and interest of approximately $30 million. On December 2, 2014, the Geneva Tribunal of First Instance entered a judgment that accepted some, and rejected other, of IRU's claims. The judgment awarded IRU CHF 16.8 million ($17 million at June 30, 2016 exchange rates) and $3.1 million, plus interest and adverse costs. The entire amount of the judgment, including interest through December 31, 2014, totaled CHF 27.9 million ($29 million at June 30, 2016 exchange rates) and $5 million. On January 26, 2015, in return for IRU agreeing not to appeal the bulk of its dismissed claims, the Aon subsidiary agreed not to appeal a part of the judgment and to pay IRU CHF 12.8 million ($13 million at June 30, 2016 exchange rates) and $4.7 million without Aon admitting liability. The Aon subsidiary appealed those aspects of the judgment it retained the right to appeal. IRU did not appeal. The Geneva Appellate Court affirmed the judgment of the Geneva Tribunal of First Instance. The Geneva Appellate Court’s judgment can be appealed to the Swiss Supreme Court. The Aon subsidiary's maximum liability on appeal is limited to CHF 8.7 million ($9 million at June 30, 2016 exchange rates) and $115,000 (plus interest and costs) beyond what the subsidiary has already paid.

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 has been filed, although a tolling agreement has been 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 ($62 million at June 30, 2016 exchange rates). In December 2014, the Court of Appeal granted the employer leave to appeal the High Court decision. The Court of Appeal hearing was set for October 2015, but has been postponed to permit the parties to discuss possible settlement. Aon believes that it has meritorious defenses and intends to vigorously defend itself against this potential 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 2010/2011 Canterbury Earthquakes. LPC claims damages of approximately NZD 184 million ($131 million at June 30, 2016 exchange rates) plus interest and costs. Aon believes that it has meritorious defenses and intends to vigorously defend itself against these claims.

In addition, from time to time, Aon's clients may bring claims and take legal action pertaining to the performance of fiduciary responsibilities. Whether client claims and legal action related to the Company's performance of fiduciary responsibilities are founded or unfounded, if such claims and legal actions are resolved in a manner unfavorable to the Company, they may adversely affect Aon's financial results and materially impair the market perception of the Company and that of its products and services.

Guarantees and Indemnifications

In connection with the redomicile of Aon's headquarters (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), (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), and (4) First Supplemental Indenture, dated as of April 2, 2012, among Aon Finance N.S. 1, ULC, as issuer, Aon Corporation, as guarantor, Aon plc, as guarantor, and Computershare Trust Company of Canada, as trustee.

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 Condensed 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.

Letters of Credit

The Company had total letters of credit ("LOCs") outstanding of approximately $82 million at June 30, 2016, compared to $58 million at December 31, 2015. 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 the Company's own workers compensation program. The Company has also issued LOCs to cover contingent payments for taxes and other business obligations to third parties, and other guarantees for miscellaneous purposes at its international subsidiaries.

Commitments

The Company has provided commitments to fund certain limited partnerships in which it has an interest in the event that the general partners request funding. Some of these commitments have specific expiration dates and the maximum potential funding under these commitments was $12 million at June 30, 2016 and December 31, 2015. During the three and six months ended June 30, 2016, no significant commitments were funded.

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 $89 million at June 30, 2016 compared to $104 million at December 31, 2015.