to net written premiums is the result of timing; premiums are earned ratably over the coverage period whereas written premiums are recorded at the initiation of the coverage period.
Hurricane Irma ($43.6 million), Hurricane Harvey ($40.7 million), the Northern California wildfires ($14.0 million), the Mexico City earthquake ($4.9 million) and Cyclone Debbie in Australia ($3.3 million).
Incurred losses and LAE decreased by 2.7% to $1,065.8 million in 2018 compared to $1,094.9 million in 2017, mainly due to a decrease of $127.5 million in current year catastrophe losses, partially offset by an increase in current year attritional losses of $105.5 million, due mainly to the impact of the increase in premiums earned. The current year catastrophe losses of $43.1 million in 2018 are outlined above. The $170.6 million of current year catastrophe losses in 2017 were due to Hurricane Irma ($75.1 million), Hurricane Harvey ($68.0 million), Hurricane Maria ($14.0 million), the 2017 US Midwest storms ($6.0 million), the Northern California wildfires ($3.0 million), the Southern California wildfires ($2.0 million), the Mexico City earthquake ($1.4 million) and Cyclone Debbie in Australia ($1.1 million).
Commission and brokerage increased by 7.0% to $267.4 million in 2018 compared to $250.1 million in 2017. The increase in 2018 was mainly due to the impact of the increase in premiums earned. Segment other underwriting expenses increased to $228.7 million in 2018 compared to $186.1 million in 2017. The increase was mainly due to the impact of the increase in premiums earned and increased expenses related to the continued build out of the insurance business.
Our carried reserves at each reporting date are management’s best estimate of ultimate unpaid losses and LAE at that date. We complete detailed reserve studies for each exposure group annually for our reinsurance and insurance operations. The completed annual reinsurance reserve studies are “rolled forward” for each accounting period until the subsequent reserve study is completed. Analyzing the roll-forward process involves comparing actual reported losses to expected losses based on the most recent reserve study. We analyze significant variances between actual and expected losses and also consider recent market, underwriting and management criteria to determine management’s best estimate of ultimate unpaid losses and LAE. As a result of these additional factors, in some instances the selected reserve level may be higher or lower than the actuarial indicated estimate.
Shareholders’ Equity. Our shareholders’ equity increased to $9,132.9 million as of December 31, 2019 from $7,860.8 million as of December 31, 2018. This increase was the result of $1,009.5 million of net income, $483.8 million of unrealized appreciation on investments net of tax, $30.9 million of share-based compensation transactions and $14.0 million of net foreign currency translation adjustments, partially offset by $234.3 million of shareholder dividends, the repurchase of 0.1 million common shares for $24.6 million and $7.1 million of net benefit plan obligation adjustments, net of tax.
Capital. Shareholders’ equity at December 31, 2019 and December 31, 2018 was $9,132.9 million and $7,860.8 million, respectively. Management’s objective in managing capital is to ensure its overall capital level, as well as the capital levels of its operating subsidiaries, exceed the amounts required by regulators, the amount needed to support our current financial strength ratings from rating agencies and our own economic capital models. The Company’s capital has historically exceeded these benchmark levels.
In addition to our cash flows from operations and liquid investments, we also have multiple credit facilities that provide up to $200.0 million of unsecured revolving credit for liquidity but more importantly provide for up to $600.0 million and £47.0 million of collateralized standby letters of credit to support business written by our Bermuda operating subsidiaries.
The Everest International Credit Facility requires Group to maintain a debt to capital ratio of not greater than 0.35 to 1 and to maintain a minimum net worth. Minimum net worth is an amount equal to the sum of $5,326.0 million (70% of consolidated net worth as of December 31, 2015), plus 25% of consolidated net income for each of Group’s fiscal quarters, for which statements are available ending on or after January 1, 2015 and for which net income is positive, plus 25% of any increase in consolidated net worth of Group during such period attributable to the issuance of ordinary and preferred shares, which at December 31, 2019, was $6,246.8 million. As of December 31, 2019, the Company was in compliance with all Everest International Credit Facility requirements.
Management estimates that the projected net economic loss from its largest 100-year event in a given zone represents approximately 6% of its December 31, 2019 shareholders’ equity. Economic loss is the PML exposure, net of third party reinsurance, reduced by estimated reinstatement premiums to renew coverage and estimated income taxes. The impact of income taxes on the PML depends on the distribution of the losses by corporate entity, which is also affected by inter-affiliate reinsurance. Management also monitors and controls its largest PMLs at multiple points along the loss distribution curve, such as loss amounts at the 20, 50, 100, 250, 500 and 1,000 year return periods. This process enables management to identify and control exposure accumulations and to integrate such exposures into enterprise risk, underwriting and capital management decisions.
Interest rate risk is the potential change in value of the fixed maturity securities portfolio, including short-term investments, from a change in market interest rates. In a declining interest rate environment, it includes prepayment risk on the $3,048.8 million of mortgage-backed securities in the $16,830.8 million fixed maturity portfolio. Prepayment risk results from potential accelerated principal payments that shorten the average life and thus the expected yield of the security.