•major public health issues, and specifically the COVID-19 pandemic and the resulting impacts on economic conditions and financial markets;
Life insurance companies are subject to NAIC RBC requirements and rating agencies utilize a form of RBC to partially determine capital strength of insurance companies. Our RBC ratio at both December 31, 2020 and December 31, 2019 was 372%.
We coinsure 80% of the annuity deposits received from certain multi-year rate guaranteed annuities and 20% of certain fixed index annuities sold by Eagle Life through broker/dealers and banks. The decrease in coinsurance ceded premiums in 2020 was attributable to a decrease in certain multi-year rate guaranteed annuities and fixed index annuities sold by Eagle Life during 2020 compared to 2019.
Non-GAAP operating income available to common stockholders, a non-GAAP financial measure (see reconciliation to net income available to common stockholders in Item 6. Selected Consolidated Financial Data) decreased 87% to $69.1 million in 2020 and increased 29% to $548.2 million in 2019 from $425.7 million in 2018. The decrease in non-GAAP operating income available to common stockholders for the year ended December 31, 2020 was primarily a result of the impact of assumption updates made during 2020 compared to the impact of assumption updates made during 2019. Non-GAAP operating income available to common stockholders and Non-GAAP operating income available to common stockholders per common share - assuming dilution, excluding the impact of assumption updates, for the year ended December 31, 2020 were $410.0 million and $4.44 per share, respectively. Non-GAAP operating income available to common stockholders and Non-GAAP operating income available to common stockholders per common share - assuming dilution, excluding the impact of assumption updates, for the year ended December 31, 2019 were $424.4 million and $4.62 per share, respectively.
Non-GAAP operating income available to common stockholders for the year ended December 31, 2020 was negatively impacted by a decrease in the aggregate investment spread as previously noted. Non-GAAP operating income available to common stockholders for the year ended December 31, 2020 was positively impacted by $30.0 million related to the provision of the CARES ACT which allowed net operating losses for 2018 through 2020 to be carried back to previous tax years in which a 35% statutory tax rate was in effect.
Net realized gains (losses) on investments include gains and losses on the sale of securities and other investments and credit losses on our securities and mortgage loans on real estate. Net realized gains (losses) on investments fluctuate from year to year primarily due to changes in the interest rate and economic environment and the timing of the sale of investments. See Note 3 - Investments and Note 4 - Mortgage Loans on Real Estate to our audited consolidated financial statements and Financial Condition - Credit Losses for a detailed presentation of the types of investments that generated the gains (losses) as well as discussion of credit losses on our securities recognized during the periods presented and Financial Condition - Investments and Note 4 - Mortgage Loans on Real Estate to our audited consolidated financial statements for discussion of credit losses recognized on mortgage loans on real estate.
Commercial mortgage backed securities: The commercial mortgage backed securities included on the watch list have generally experienced higher levels of stress due to the impact COVID-19 is having on the economy. As a result of our process for identifying securities that could potentially have credit losses and our intent to sell certain commercial mortgage backed securities, we recognized credit losses of $29.2 million on these securities during 2020.
Several factors led us to believe that full recovery of amortized cost is not expected on the securities for which we recognized credit losses. A discussion of these factors, our policy and process to identify securities that could potentially have credit loss is presented in Note 3 - Investments to our audited consolidated financial statements in this Form 10-K, which is incorporated by reference in this Item 7.
See Note 9 - Notes Payable and Amounts Due Under Repurchase Agreements to our audited consolidated financial statements in this Form 10-K, which is incorporated by reference in this Item 7 for discussion of our notes and loan payable and borrowings under repurchase agreements.
Level 1 -Quoted prices are available in active markets for identical financial instruments as of the reporting date. We do not adjust the quoted price for these financial instruments, even in situations where we hold a large position and a sale could reasonably impact the quoted price.
The following table presents the fair value of fixed maturity securities, available for sale, by pricing source and hierarchy level as of December 31, 2020 and 2019, respectively:
As of December 31, 2020, we utilized an estimate of 2.10% for the expected cost of annual call options, which is based on estimated long-term account value growth and a historical review of our actual options costs. If the expected cost of annual call options we purchase in the future to fund index credits beyond the next policy anniversary date were to increase by 25 basis points, our reserves for fixed index annuities would increase by $647.2 million. A decrease of 25 basis points in the expected cost of annual call options would decrease our reserves for fixed index annuities by $610.7 million.
Our assumptions related to the percentage of policyholders who elect to receive lifetime income benefit payments is based on actual experience and our outlook as to future expectations for utilization rates. If the percentage of policyholders who elect to receive lifetime income benefit payments was increased by 10% at December 31, 2020, our liability for lifetime income benefit riders would increase by $79.8 million. A decrease by 10% in the percentage of policyholders who elect to receive lifetime income benefit payments would decrease our liability for lifetime income benefit riders by $58.5 million.
The net investment earned rate is a key component in the calculation of expected assessments in the projection period. The net investment earned rate is based on current yields being earned in our invested assets portfolio, future spot rates, the expected mean reversion period and expected spread we will earn above the risk-free rate. If the net investment earned rate were to increase 10 basis points, our liability for lifetime income benefit riders would decrease by $23.4 million. A decrease in the net investment earned rate of 10 basis points would increase our liability for lifetime income benefit riders by $24.0 million.