Adjusted operating margin, a non-GAAP measure defined under the caption “Review of Consolidated Results - Adjusted Operating Margin,” was 27.5% in 2019, compared to 25.0% in the prior year. The increase in adjusted operating margin primarily reflects organic revenue growth of 6%, operational improvement, and $169 million of incremental savings related to restructuring and other operational improvement initiatives.
Interest income represents income earned on operating cash balances and other income-producing investments. It does not include interest earned on funds held on behalf of clients. Interest income was $8 million in 2019, an increase of $3 million, or 60%, from 2018, reflecting the currency composition of operating cash.
Fiduciary investment income for the years ended December 31, 2019, 2018, and 2017, respectively, was $74 million, $53 million, and $32 million.
The Restructuring Plan resulted in cumulative charges of $1,433 million, consisting of $619 million in workforce reduction, $119 million in technology rationalization costs, $69 million in lease consolidation costs, $53 million in non-cash asset impairments, and $573 million in other costs, including certain separation costs associated with the sale of the Divested Business. These charges are included in Compensation and benefits, Information technology, Premises, Depreciation of fixed assets, and Other general expense in the accompanying Consolidated Statements of Income. Before any potential reinvestment of savings, the Restructuring Plan delivered annual cumulative expense savings of $529 million in 2019 and is expected to deliver run-rate savings of $580 million annually in 2020. The Company eliminated 5,832 positions under the Restructuring Plan.
Other costs associated with the Restructuring Plan include those to separate the Divested Business, as well as moving costs, and consulting and legal fees. These costs are typically recognized when incurred.
Additionally, we are 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, we would be required to fulfill the remaining contract terms, which expire on various dates through 2023. As of December 31, 2019, the undiscounted maximum potential future payments under the performance guarantees were $151 million, with an estimated fair value of $1 million. No cash payments were made in connection to the performance guarantees during the year ended December 31, 2019.
Operating leases are included in Operating lease ROU assets, Other current liabilities, and Non-current operating lease liabilities on the Consolidated Statements of Financial Position. Finance leases are included in Other non-current assets, Other current liabilities, and Other non-current liabilities on the Consolidated Statements of Financial Position.
As a result of applying the modified retrospective approach to adopt the new leasing standard, the following adjustments were made to the Consolidated Statement of Financial Position as of January 1, 2019 (in millions):
In December 2019, the FASB issued new accounting guidance that simplifies the accounting for income taxes by eliminating some exceptions to the general approach in the existing guidance. It also clarifies certain aspects of the existing guidance to promote more consistent application. The new guidance is effective for Aon in the first quarter of 2021, with early adoption permitted. The Company is currently evaluating the impact that the guidance will have on the Consolidated Financial Statements and the period of adoption.
In August 2018, the FASB issued new accounting guidance related to the disclosure requirements for employers that sponsor defined benefit pension and other postretirement benefit plans. The guidance requires sponsors of these plans to provide additional disclosures, including weighted average interest rates used in the entity’s cash balance pension plans and a narrative description of reasons for any significant gains or losses impacting the benefit obligation for the period, and eliminates certain previous disclosure requirements. The new guidance is effective for Aon in the first quarter of 2021 with early adoption permitted and will be applied retrospectively. The Company is currently evaluating the impact that the guidance will have on the Consolidated Financial Statements and the period of adoption.
The Company performs a reconciliation of the income tax provisions based on its domicile and statutory rate at each reporting period. The 2019, 2018, and 2017 reconciliations are based on the U.K. statutory corporate tax rate of 19.0%, 19.0%, and 19.3%, respectively. The reconciliation to the provisions from continuing operations reflected in the Consolidated Financial Statements is as follows:
Service cost is reported in Compensation and benefits and all other components are reported in Other income (expense) as follows (in millions):
Selected quarterly financial data for the years ended December 31, 2019 and 2018 are as follows (in millions, except per share data):