The net loss ratio in our commercial lines of business, excluding assumed reinsurance, was 77.8 percent in 2020 compared to 75.6 percent in 2019 and 71.4 percent in 2018. The net loss ratio in 2020 increased compared to 2019 primarily due to an increase in catastrophe losses. The net loss ratio in 2019 increased compared to 2018 with a deterioration in commercial auto and other liability lines of business from an increase in severity of commercial auto losses and auto related bodily injury claims. Also contributing to the deterioration in 2019 compared to 2018, was an increase in commercial fire and allied lines of business due to an increase in catastrophe losses.
We have exposure to construction defect liabilities in Colorado and surrounding states. We have historically insured small- to medium-sized contractors in this geographic area. In an effort to limit the number of future claims from multi-unit buildings, we implemented policy exclusions in 2009, later revised in 2010, that exclude liability coverage for contractors performing "residential structural" operations on any building project with more than 12 units or on single family homes in any subdivision where the contractor is working on more than 15 homes. The exclusions do not apply to remodeling or repair of an existing structure. We also changed our underwriting guidelines to add a professional liability exclusion when contractors prepare their own design work or blueprints and implemented the multi-family exclusion and tract home building limitation form for the state of Colorado and our other western states as a means to reduce our exposure in future years. When offering commercial umbrella coverage for structural residential contractors, limits of liability are typically limited to a maximum of $2.0 million per occurrence. Requests to provide additional insured status for "developers" are declined.
Due to our determination that we may not be able to fully realize the benefits of the net operating loss ("NOL") acquired in the purchase of American Indemnity Financial Corporation in 1999, which are only available to offset the future taxable income of our property and casualty insurance operations and are further limited as to the amount that can be utilized in any given year, we have recorded a valuation allowance against these NOLs. Based on a yearly review, we determine whether the benefit of the NOLs can be realized, and, if so, the decrease in the valuation allowance is recorded as a reduction to current federal income tax expense. If NOLs expire during the year, the decrease in the valuation allowance is offset with a corresponding decrease to the deferred income tax asset. The valuation allowance was reduced by $0.7 million in 2018 due to the realization of $3.1 million in NOLs, therefore reducing the valuation allowance to zero. During 2018, the remaining NOL from the purchase of American Indemnity Financial Corporation was fully realized.
The ensuing sell-off was sharp. Over the past 30 years the S&P 500 Index has only produced double-digit negative weekly returns five times. Two of these weeks occurred during a four-week span from February through March of 2020. The week ended March 20, 2020 was the third worst weekly performance for the S&P 500 Index since its inception. Over the same period, the spread on investment grade corporate bonds more than tripled while interest rates plummeted. The selling became so widespread that even Treasuries began to lose stability. The price on the 10-year note surged in late February as rates dropped 70 basis points. Less than 10 days later, prices collapsed as rates reversed course and surged 70 basis points. Rates dropped precipitously again and began to stabilize after historic and decisive action taken by the Federal Reserve ("The Fed"). The Fed entered the market boldly in mid-March, activating multiple funding facilities, some for the first time ever, and pledging to buy certain financial assets with no limit, including corporate and municipal debt in either primary or secondary markets.
We monitor our capital adequacy to support our business on a regular basis. The future capital requirements of our business will depend on many factors, including our ability to write new business successfully and to establish premium rates and reserves at levels sufficient to cover losses. Our ability to underwrite is largely dependent upon the quality of our claims paying and financial strength ratings as evaluated by independent rating agencies. In particular, we require (1) sufficient capital to maintain our financial strength ratings, as issued by various rating agencies, at a level considered necessary by management to enable our insurance company subsidiaries to compete and (2) sufficient capital to enable our insurance company subsidiaries to meet the capital adequacy tests performed by regulatory agencies in the United States.
As a holding company with no independent operations of its own, United Fire Group, Inc. relies on dividends received from its insurance company subsidiaries in order to pay dividends to its common shareholders. Dividends payable by our insurance subsidiaries are governed by the laws in the states in which they are domiciled. In all cases, these state laws permit the payment of dividends only from earned surplus arising from business operations. For example, under Iowa law, the maximum dividend or distribution that may be paid within a 12-month period without prior approval of the Iowa Insurance Commissioner is generally restricted to the greater of 10 percent of statutory surplus as of the preceding December 31, or net income of the preceding calendar year on a statutory basis, not greater than earned statutory surplus. Other states in which our insurance company subsidiaries are domiciled may impose similar restrictions on dividends and distributions. Based on these restrictions, at December 31, 2020, our insurance company subsidiary, United Fire & Casualty, is able to make a maximum of $66.8 million in dividend payments without prior regulatory approval. These restrictions are not expected to have a material impact in meeting our cash obligations.
The amounts presented are estimates of the dollar amounts and time periods in which we expect to pay out our gross loss and loss settlement expense reserves. Because the timing of future payments may vary from the stated contractual obligation, these amounts are estimates based upon historical payment patterns and may not represent actual future payments. Refer to "Critical Accounting Policies - Losses and Loss Settlement Expenses" in this section for further discussion.
costs in the period such changes in assumptions are made. The premium deficiency charge calculated for the quarter ended December 31, 2020 was $1.5 million compared to the premium deficiency charge of $4.6 million calculated for the same period of 2019.
Due to the formula-based nature of our IBNR and loss settlement expense reserve calculations, changes in the key assumptions utilized to generate these reserves can impact our reported results. It is not possible to isolate and measure the potential impact of just one of these factors, and future loss trends could be partially impacted by all factors concurrently. Nevertheless, it is meaningful to view the sensitivity of the reserves to potential changes in these variables such as claim frequency and severity. To demonstrate the sensitivity of reserves to changes in significant assumptions, the following example is presented. The amounts reflect the pre-tax impact on earnings from a hypothetical percentage change in the calculation of IBNR and loss settlement expense reserves at December 31, 2020. The impact to pre-tax earnings would be a decrease if the reserves were to be adjusted upwards and an increase if the reserves were to be adjusted downwards. We believe that the changes presented are reasonably likely based upon an analysis of our historical IBNR and loss settlement expense reserve experience.
A change in any one or more of these assumptions is likely to result in an ultimate liability different from the original actuarial estimate. Such changes in estimates may be material. For example, a 100 basis point decrease in our estimated discount rate would increase the pension and post-retirement benefit obligation at December 31, 2020, by $51.1 million and $5.9 million, respectively, while a 100 basis point increase in the rate would decrease the benefit obligation at December 31, 2020, by $39.8 million and $4.6 million, respectively.
The Company's quantitative goodwill impairment test involved estimating the fair value of the Company, which was sensitive to significant assumptions, such as forecasted revenues and loss and loss settlement expenses, discount rate, and terminal growth rate which are used in the income approach and comparable publicly traded companies and estimated valuation multiples which are used in the market approach.
In January 2017, the FASB issued new guidance which simplifies the test for goodwill impairment. The new guidance eliminates the implied fair value calculation when measuring a goodwill impairment charge. Under the new guidance, impairment charges are based on the excess of the carrying value over fair value of goodwill. The new guidance was effective for annual and interim periods beginning after December 15, 2019. The Company adopted the new guidance as of January 1, 2020. The adoption did not have a significant impact on the Company's financial position or results of operations.
Net investment income for the years ended December 31, 2020, 2019 and 2018, is comprised of the following:
The following tables provide information about incurred and paid losses and loss settlement expense development as of December 31, 2020, net of reinsurance, as well as cumulative development, cumulative claim frequency and IBNR liabilities. Claim data for Mercer Insurance Group, Inc., which was acquired on March 28, 2011, is presented retrospectively.
We have audited the accompanying consolidated balance sheets of United Fire Group, Inc. (the Company) as of December 31, 2020 and 2019, and the related consolidated statements of income, comprehensive income, stockholders' equity and cash flows for each of the three years in the period ended December 31, 2020, and the related notes and financial statement schedules listed in the Index at Item 15(a)(2) (collectively referred to as the “financial statements”). In our opinion, the financial statements referred to above present fairly, in all material respects, the consolidated financial position of the Company at December 31, 2020 and 2019, and the consolidated results of its operations and its cash flows for each of the three years in the period ended December 31, 2020, in conformity with U.S. generally accepted accounting principles.