Chaucer Catastrophe events - the most significant events of 2018 were typhoons Jebi and Trami, the California wildfires, and hurricanes Michael and Florence. For these events, which represent 71% of the total 2018 accident year incurred catastrophe losses, a 25% increase in their estimated gross ultimate claims deterioration would have increased incurred catastrophe losses by approximately $6 million, net of reinsurance. For these events, a 25% decrease in their estimated gross ultimate claims deterioration would have decreased incurred catastrophe losses by approximately $9 million, net of reinsurance.
Net investment income increased $2.9 million for the year ended December 31, 2018 and $6.3 million for the year ended December 31, 2017 as compared to the prior years. The increase in 2018 was primarily due to the impact of higher new money yields and higher income on certain reinsurance contracts subject to deposit accounting, partially offset by lower average invested assets. The increase in 2017 compared to 2016 also reflects higher income on certain reinsurance contracts subject to deposit accounting, as well as the impact of higher operational cashflows.
The income tax provision on the Chaucer business was an expense of $4.7 million, $21.7 million and $37.2 million for 2018, 2017 and 2016, respectively. The provision for 2017 reflects a one-time expense of $26.2 million related to the enactment of the TCJA. This expense included a revaluation of our net deferred tax assets and the recognition of federal income tax on previously untaxed earnings from foreign operations of $13.5 million and $12.7 million, respectively. Absent these items, the provision for income tax for 2017 would have been a benefit of $4.5 million.
indefinitely reinvested overseas and was not subject to U.S. tax until repatriated. These non-U.S. earnings are now subject to a one-time mandatory toll charge totaling $12.7 million, which is recorded within discontinued operations as a component of income tax expense related to the Chaucer business for the year ended December 31, 2017.
We currently have a U.S. qualified defined benefit plan and several smaller non-qualified benefit plans. Additionally, we held a defined benefit pension plan for our international subsidiary, Chaucer, until its sale on December 28, 2018. Accordingly, this discussion focuses on our domestic plans only. In order to measure the liabilities and expense associated with these plans, we must make various estimates and key assumptions, including discount rates used to value liabilities, assumed rates of return on plan assets, employee turnover rates and anticipated mortality rates. These estimates and assumptions are reviewed at least annually and are based on our historical experience, as well as current facts and circumstances. In addition, we use outside actuaries to assist in measuring the expenses and liabilities associated with our defined benefit pension plan.
Dividends to common shareholders are subject to quarterly board approval and declaration. During 2018, as declared by the Board, dividends totaled $288.0 million. This included three quarterly dividends of $0.54 per share and one quarterly dividend of $0.60 per share to our shareholders, as well as a special dividend declared on December 30, 2018 of $4.75 per share. We believe that our holding company assets are sufficient to provide for future shareholder dividends, should the Board of Directors declare them.
Catastrophe loss ratio -The ratio of catastrophe losses incurred to premiums earned.
Net investment income includes interest, dividends, and income from limited partnership interests. Interest income is recognized based on the effective yield method which includes the amortization of premiums and accretion of discounts. The effective yield used to determine the amortization for fixed maturities subject to prepayment risk, such as mortgage-backed and asset-backed securities, is recalculated and adjusted periodically based upon actual historical and projected future cash flows. The adjustment to yields for highly rated prepayable fixed maturities is accounted for using the retrospective method. The adjustment to yields for all other prepayable fixed maturities is accounted for using the prospective method. Fixed maturities and mortgage participations that are delinquent are placed on non-accrual status, and thereafter interest income is recognized only when cash payments are received.
At December 31, 2018, scheduled maturities of mortgage participations and other mortgage loans were as follows: due in 2020 - $10.0 million; in 2021 - $57.4 million; 2022 - $28.1 million; 2023 - $17.0 million and thereafter - $293.2 million. There were no scheduled loan maturities in 2019. Actual maturities could differ from contractual maturities because borrowers may have the right to prepay obligations with or without prepayment penalties and loans may be refinanced. During 2018, the Company did not refinance any loans based on terms that differed from current market rates.
At December 31, 2018, the Company had a $200.0 million credit agreement which expires in May 2019. The Company had no borrowings under this agreement as of December 31, 2018.
Interest expense was $45.1 million, $45.2 million, and $51.4 million in 2018, 2017 and 2016, respectively. At December 31, 2018, the Company was in compliance with the covenants associated with all of its debt indentures and credit arrangements.
Weighted average assumptions used to determine pension benefit obligations are as follows:
In addition to the defined benefit plans, THG provides a qualified defined contribution 401(k) plan for its U.S. employees, whereby the Company matches employee elective 401(k) contributions, up to a maximum of 6% of eligible compensation in 2018, 2017, and 2016. The Company’s expense for this matching provision was $21.7 million, $21.3 million and $21.4 million for 2018, 2017, and 2016, respectively. In addition to this matching provision, the Company can elect to make an annual contribution to employees’ accounts. Additional contributions amounted to $2.2 million and were contributed to the plan during 2017.
Compensation cost, excluding the discontinued operations related to the Chaucer business, for the years ended December 31, 2018, 2017, and 2016 totaled $14.9 million, $10.5 million and $10.5 million, respectively. Related tax benefits were $3.1 million, $3.7 million and $3.7 million, respectively. Compensation cost attributable to the Chaucer business, net of taxes, for the years ended December 31, 2017 and 2016 totaled $1.2 million and $1.0 million, respectively, and were reflected in discontinued operations.
Rental expenses for operating leases related to the former Chaucer business amounted to $3.8 million, $3.3 million and $2.9 million in 2018, 2017 and 2016, respectively. These expenses relate primarily to building leases of Chaucer and are included in discontinued operations.