the fourth quarter of 2020, we reversed the remainder of the allowance for credit losses that we had recognized in the previous quarters of 2020. During the remainder of 2020, we did not experience any credit losses exceeding $10 million. We had one individual realized investment loss of $20.8 million from impairments during 2019. We had no individual realized investment losses of $10.0 million or greater from impairments during 2018. We had no individual realized investment losses of $10.0 million or greater from sales of fixed maturity securities during 2020. We had one individual realized investment loss of $15.6 million from the sale of fixed maturity securities during 2019, and we had no individual realized investment losses of $10.0 million or greater from the sale of fixed maturity securities during 2018. At December 31, 2020, our mortgage/asset-backed securities had an average life of 3.8 years, effective duration of 2.7 years, and a weighted average credit rating of AAA. The mortgage/asset-backed securities are valued on a monthly basis using valuations supplied by the brokerage firms that are dealers in these securities as well as independent pricing services. One of the risks involved in investing in mortgage/asset-backed securities is the uncertainty of the timing of cash flows from the underlying loans due to prepayment of principal with the possibility of reinvesting the funds in a lower interest rate environment. We use models which incorporate economic variables and possible future interest rate scenarios to predict future prepayment rates. The timing of prepayment cash flows may also cause volatility in our recognition of investment income. We recognize investment income on these securities using a constant effective yield based on projected prepayments of the underlying loans and the estimated economic life of the securities. Actual prepayment experience is reviewed periodically, and effective yields are recalculated when differences arise between prepayments originally projected and the actual prepayments received and currently projected. The effective yield is recalculated on a retrospective basis, and the adjustment is reflected in net investment income. As of December 31, 2020, the amortized cost net of allowance for credit losses and fair value of our below-investment-grade fixed maturity securities was $3,256.0 million and $3,500.1 million, respectively. Below-investment-grade securities are inherently riskier than investment-grade securities since the risk of default by the issuer, by definition and as exhibited by bond rating, is higher. Also, the secondary market for certain below- investment-grade issues can be highly illiquid. Additional downgrades may occur, but we do not anticipate any liquidity problems resulting from our investments in below-investment-grade securities, nor do we expect these investments to adversely affect our ability to hold our other investments to maturity. Our investment portfolio has exposure to companies whose businesses are negatively impacted by lower oil and natural gas prices. These include exploration and production companies, refineries, midstream pipeline companies, and oilfield service businesses. The sharp decline in energy prices and decrease in demand due to COVID-19, which began to occur in the first quarter of 2020, put pressure on the earnings and cash flows of these businesses. Oil prices did improve during the remainder of 2020; however, we continue to monitor this sector closely. The degree to which a business is affected by energy prices can vary greatly depending on, among other things, its energy subsector, geographic locations, cost structure flexibility, capital structure, and hedging policies. At December 31, 2020, approximately 51 percent of our exposure to the energy sector was represented by the midstream (pipeline) subsector which tends to be more correlated to product volume sales than to commodity prices. Approximately 24 percent of our exposure is in the oil and gas independent exploration and production subsector where underlying profitability is highly correlated with oil and gas prices. Approximately 16 percent of our exposure is in the integrated subsector which is comprised of large highly rated companies. The majority of our energy sector holdings are investment-grade fixed maturity securities. 89 Fixed Maturity Securities - Energy Sector