In the fourth quarter, we grew gross written premiums by 13% and net written premiums by 16%, with strong growth across both segments.
Our underlying combined ratio was 86.3%, a 4 point improvement over the fourth quarter of 2019, with both segments showing significant improvement in loss and expense ratios.
Net investment income was very strong at $222 million compared to $146 million in the prior-year quarter.
For 2020, Everest grew gross written premiums of 15% and net written premiums 17% year-over-year.
We delivered $514 million in net income and $300 million in operating income despite the COVID-19 loss provision, the prior year reserve strengthening, and an active Cat year.
Our dividend adjusted book value per share grew over 11%.
The underlying combined ratio improved almost a point to 87.5% year-over-year, with our insurance segment improving 2.3 points to 94.2%.
Underwriting profitability remains at the core of everything that we do As previously announced, in the fourth quarter, we strengthened prior accident year reserves in our Reinsurance segment by $400 million.
In the fourth quarter, we also added $76 million primarily for third-party lines to our COVID-19 loss provision.
Despite a high frequency of storms in the fourth quarter, our manageable catastrophe losses of $70 million resulted from disciplined underwriting and the purposeful reduction of volatility over the last two years in our reinsurance portfolio.
Gross written premiums grew 12% in the quarter and 15% in 2020.
The attritional combined ratio ex-COVID was 83.9%, an improvement from 87.4% in the prior fourth quarter.
Gross written premiums grew 15% or 18%, excluding terminated programs, with gross written premium of $872 million in the quarter and over $3.2 billion for 2020.
Everest Insurance delivered an improved attritional combined ratio of 93.8% for the fourth quarter, a 4.3 point improvement over the fourth quarter of 2019 and 94.2% for the full year 2020, a 2.3 point improvement over 2019.
We achieved record renewal rate increases of 21% in the fourth quarter, excluding workers' compensation, and up 14% including workers' compensation, where we are seeing rates flatten.
Consistent with prior quarters, these increases are led by property, up 21%; excess casualty, up 50%; D&O, up 35%; and commercial auto, up 17%.
We are also seeing widespread increases in other lines of business, which had been slower to turn, most notably, general liability, now up 9%.
The positive quarterly net income result was achieved despite a prior year reserve strengthening charge of $400 million, a COVID provision of $76 million, and catastrophe losses of $70 million.
Everest reported net income of $64 million for the quarter and $514 million for the year, resulting in a return on equity of 5.8% for 2020.
We had a $44 million operating loss for Q4, given the charges, and generated an operating income of $300 million for the year.
Our net income in the quarter reflect strong investment income performance and improved attritional loss and combined ratios, offset by Cat, COVID and reserve charges, the catastrophe losses of $70 million are pre-tax and net of reinsurance, with $60 million from reinsurance and $10 million from insurance, driven by hurricane Delta, Zeta, and the Australian Queensland hailstorm.
The estimate implied market share of industry losses, is just over 60 basis points for Everest.
Year-to-date, the results include catastrophe losses of $425 million compared to $576 million during 2019.
In the fourth quarter, we added $76 million to our COVID loss provision, reflecting the ongoing nature of this event and our consistent reserving philosophy.
This amount includes $56 million in the Reinsurance segment and $20 million in the insurance segment, and is in addition to the $435 million of pandemic losses estimated in the first nine months of 2020.
For the full year 2020, the total pandemic loss provision is $511 million, of which more than 80% is classified as IBNR.
Everest had an underwriting loss in Q4 of $219 million due to the prior year reserve adjustment charge as compared to an underwriting loss of $29 million for Q4 2019.
As Juan mentioned, we booked $400 million prior year reserve strengthening in the fourth quarter exclusively for the Reinsurance Division, primarily within long tail casualty segments, such as GL, auto liability, and professional lines for accident years 2015 through 2018.
Turning to Everest's market position and growth on a year-to-date basis, gross written premium was $10.
5 billion, up $1.3 billion or 15% compared to 2019.
This reflects strong and diversified growth in both segments with reinsurance up 15% and insurance up 15% compared to 2019.
Our underlying attritional loss and combined ratios are strong and improving, excluding the catastrophe losses and impact from the COVID-19 pandemic, the attritional combined ratio was 87.5% for 2020 compared to 88.4% for 2019.
Excluding the pandemic loss estimate, the group attritional loss ratio for 2020 was 60.1%, down from 60.2% for 2019, with insurance improving from 66% the 64.8%.
For reinsurance, the 2020 attritional combined ratio, excluding the pandemic loss estimate and prior year reserve charge was 85.2%, down from 85.5% in 2019.
For insurance, the 2020 attritional combined ratio, excluding the pandemic loss estimate was 94.2% compared to 96.5% in 2019.
The group commission ratio of 21.6% year-to-date was down from 23% in 2019, largely due to business mix, a one-time significant contingent commission in the Reinsurance segment during 2019, and higher ceding commission in the Insurance segment.
The group expense ratio remains low at 5.8% for 2020 versus 6% for 2019, as we benefited from premium growth and continued focus on expense management.
Q4 investment income had a strong performance of $222 million compared to $146 million for Q4 2019.
For the full year, pre-tax investment income was $642 million versus $647 million for 2019.
The fixed income portfolio generated $542 million of investment income year-to-date compared to $520 million for the same period last year.
Limited partnerships recorded $91 million of income quarter-to-date, largely due to fair market value adjustments.
Invested assets grew 23% to $25.4 billion versus $20.7 billion last year end.
This strong invested asset growth was due to $2.9 billion of operating cash flow and the proceeds of our debt issue.
The pre-tax yield to maturity on the investment portfolio was just under 3%, down from 3.4% one year ago.
Approximately 80% of our invested assets are comprised of a well diversified, high credit quality bond portfolio with duration of 3.6 years.
Our effective tax rate on operating income for 2020 was 7.7% and 12.1% on net income.
For 2021, we expect our tax rate to be approximately 12%, which reflects an annual Cat load of about 6 points of loss ratio.
Everest generated record operating cash flows of $2.9 billion compared to $1.9 billion in 2019, reflecting the strength of our growing premiums in 2020 year-over-year and a more modest level of claims paid.
Shareholders' equity was $9.7 billion at year-end 2020, up from $9.1 billion at year-end 2019.
Net book value per share stood at $243.25, up 11% versus year-end 2019, adjusted for dividends.
Everest's strong balance sheet was further strengthened by the 30-year $1 billion senior notes offering completed in early October 2020.
This is long-term capital for Everest and enhances the efficiency of our capital structure, with our debt leverage now standing at 16.4%.

Summary:
In the fourth quarter, we grew gross written premiums by 13% and net written premiums by 16%, with strong growth across both segments.
Everest reported net income of $64 million for the quarter and $514 million for the year, resulting in a return on equity of 5.8% for 2020.