2020, compared to $6.4 billion at December 31, 2019, with the increase due primarily to a decline in U.S. Treasury rates. The earned book yield on our investment portfolio was 4.75 percent for 2020 compared to a yield of 5.00 percent for 2019.
We believe our capital and financial positions are strong. At December 31, 2020, the RBC ratio for our traditional U.S. insurance subsidiaries, calculated on a weighted average basis using the NAIC Company Action Level formula, was approximately 365 percent, which is in line with our expectations. We did not repurchase shares during 2020. Our weighted average common shares outstanding, assuming dilution, equaled 203.8 million for 2020 compared to 209.9 million for 2019. As of December 31, 2020, Unum Group and our intermediate holding companies had available holding company liquidity of $1,512 million that was held primarily in fixed maturity securities, short-term investments, and cash.
•Cost of reinsurance, or prepaid reinsurance premium, of $815.7 million related to the DLR cohort, of which we recognized amortization expense of $2.6 million, or $2.0 million after tax, subsequent to the closing of the transaction.
During the fourth quarter of 2020, we completed our annual review of policy reserve adequacy, which incorporated our most recent experience and included a review of all assumptions. Based on our analysis, during the fourth quarter of 2020, we updated our reserve assumptions and determined that our policy and claim reserves should be increased by $17.5 million to reflect our updated discount rate assumptions.
On March 11, 2020, the World Health Organization identified the spread of COVID-19 as a pandemic. COVID-19 has caused significant disruption to the global economy and has unfavorably impacted our company as well as the overall insurance industry. Due to the unprecedented nature of these events and the current pace of change in this environment, we cannot fully estimate the ultimate impact of the COVID-19 pandemic at this time. We are closely monitoring several key factors related to our business that have and may continue to have adverse impacts.
In establishing policy reserves, we use assumptions that reflect our best estimate while considering the potential for adverse variances in actual future experience, which results in a total policy reserve balance that has an embedded reserve for adverse deviation. We do not, however, establish an explicit and separate reserve as a provision for adverse deviation from our assumptions.
Investment risk is measured and monitored on an ongoing basis through annual liability measurements, periodic asset/liability studies, and quarterly investment portfolio reviews. Risk tolerance is established through consideration of plan liabilities, plan funded status, and corporate financial condition. We believe our investment portfolios are well diversified by asset class and sector, with no undue risk concentrations in any one category. See Note 9 of the "Notes to Consolidated Financial Statements" contained herein in Item 8 for further discussion of the investment portfolios for our plans.
Our effective income tax rate for 2020 was 17.7 percent, compared to 20.4 percent in 2019 and 16.6 percent in 2018. Our 2020, 2019, and 2018 effective tax rates differed from the U.S. statutory rate of 21 percent due to favorable tax credits, with additional favorable adjustments in 2018 related to our prior year tax return. Also impacting the difference between the effective tax rate and the U.S. statutory rate in 2020 was the unfavorable impact of the U.K. tax rate increase, as well as the favorable impact related to net operating loss carryback. See Note 7 in the "Notes to Consolidated Financial Statements" contained herein in Item 8 for further discussion.
We had total goodwill of $45.3 million for the Unum International segment at December 31, 2020, of which, $40.2 million is attributed to the Unum UK reporting unit and $5.1 million is attributed to the Unum Poland reporting unit. Fair value of our reporting units is estimated using a combination of the income and market approaches and the key assumptions used are projected earnings and discount rate. To the extent that the future profitability of these reporting units deteriorates from current assumptions, the goodwill related to the reporting units could be at risk for impairment.
Given the uncertainty caused by the COVID-19 pandemic, we expect to experience further disruption in our sales activity, persistency, and ultimately, premium income in 2021. We could also continue to experience claims volatility, particularly in our life and disability products. The lower interest rate environment will continue to have an unfavorable impact on our profit margins, and volatility in miscellaneous investment income is likely to continue. While we believe our underlying profitability will remain strong, current economic conditions and increasing competition in the voluntary workplace market are seen as external risks to achievement of our business plans. We continuously monitor key indicators to assess our risks and adjust our business plans accordingly.
loan borrowers that have requested temporary payment deferrals, but this has not resulted in a significant number of loans with deferrals or a significant impact on our net investment income. For further discussion, see "Mortgage Loans" and "Private Equity Partnerships" contained herein in this Item 7. See "Executive Summary" for further information on the impact from COVID-19 contained herein in this Item 7.
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.
Insurance regulatory restrictions do not limit the amount of dividends available for distribution from non-insurance subsidiaries except where the non-insurance subsidiaries are held directly or indirectly by an insurance subsidiary and only indirectly by Unum Group, which does not apply to our current entity structure.
Also on May 4, 2020, S&P affirmed their ratings and outlook on our rated insurance subsidiaries and issuer credit ratings on our senior debt obligations.
Deferred acquisition costs related to non-interest sensitive policies are amortized in proportion to the premium income we expect to receive over the lives of the policies. Deferred acquisition costs related to interest sensitive policies are amortized over the lives of the policies in relation to the present value of estimated gross profits from surrender charges, mortality margins, investment returns, and expense margins. Deviations from projections result in a change to the rate of amortization in the period during which such events occur. Generally, the amortization periods for these policies approximate the estimated lives of the policies.
Where applicable, gains or costs recognized on reinsurance transactions are generally deferred and amortized into earnings based upon expected future premium income for non-interest sensitive insurance policies and estimated future gross profits for interest sensitive insurance policies. Gains or costs recognized on reinsurance transactions for non-interest sensitive products for which we no longer receive premiums are generally deferred and amortized into earnings based upon expected claim reserve patterns. The cost of reinsurance included in other assets in our consolidated balance sheet at December 31, 2020 was $813.0 million. The deferred gain on reinsurance included in other liabilities in our consolidated balance sheets at December 31, 2020 and December 31, 2019 was $5.6 million and $8.5 million, respectively.
Fair values for derivatives other than embedded derivatives in modified coinsurance arrangements are based on market quotes or pricing models and represent the net amount of cash we would have paid or received if the contracts had been settled or closed as of the last day of the period. We analyze credit default swap spreads relative to the average credit spread embedded within the London Interbank Offered Rate (LIBOR)-setting syndicate in determining the effect of credit risk on our derivatives' fair values. If net counterparty credit risk for a derivative asset is determined to be material and is not adequately reflected in the LIBOR-based fair value obtained from our pricing sources, we adjust the valuations obtained from our pricing sources. For purposes of valuing net counterparty risk, we measure the fair value of a group of financial assets and financial liabilities on the basis of the price that would be received to sell a net long position or transfer a net short position for a particular risk exposure in an orderly transaction between market participants at the measurement date under current market conditions. In regard to our own credit risk component, we adjust the valuation of derivative liabilities wherein the counterparty is exposed to our credit risk when the LIBOR-based valuation of our derivatives obtained from pricing sources does not effectively include an adequate credit component for our own credit risk.
September 2018 and the loan was considered impaired as of December 31, 2018. The loan was settled during the first quarter of 2019 resulting in an additional loss of $0.1 million. At December 31, 2020, we held no mortgage loans that were greater than 90 days past due regarding principal and/or interest payments.
In 2007, Northwind Holdings, LLC (Northwind Holdings), a wholly-owned subsidiary of Unum Group, issued $800.0 million of insured, senior secured notes, bearing interest at a floating rate equal to the three month LIBOR plus 0.78% (the Northwind notes) in a private offering.
later sell those investments for a gain. Real estate investments consist primarily of funds that hold commercial real estate investments. Alternative investments, which include private equity direct investments, private equity funds of funds, and hedge funds of funds, utilize proprietary strategies that are intended to have a low correlation to the U.S. stock market. Prohibited investments include, but are not limited to, unlisted securities, options, short sales, and investments in securities issued by Unum Group or its affiliates. The invested asset classes, asset types, and benchmark indices for our U.S. qualified defined benefit pension plan is as follows. We target approximately 36 percent to equity securities, 40 percent to fixed income securities, and 24 percent to opportunistic credits, alternative, and real estate investments.
The categorization of fair value measurements by input level for the invested assets in our U.S. pension plans is shown below. The carrying values of investment-related receivables and payables approximate fair value due to the short-term nature of the securities and are not included in the following chart. Investments valued using net asset value (NAV) as a practical expedient are not required to be categorized by input level, but these investments are included as follows to reconcile to total invested assets.
Alternative investments are valued based on NAV in a period ranging from one month to one quarter in arrears. We evaluate the need for adjustments to the NAV based on market conditions and discussions with fund managers in the period subsequent to the valuation date and prior to issuance of the financial statements. We made no adjustments to the NAV for 2020 or 2019. The private equity direct investments and private equity funds of funds generally cannot be redeemed by investors. Distributions of capital from the sale of underlying fund assets may occur at any time, but are generally concentrated between five and eight years from the formation of the fund.
Under the Stock Incentive Plan of 2012 (the 2012 Plan), which was terminated in May 2017 for the purposes of any further grants, up to 20 million shares of common stock were available for awards to our employees, officers, consultants, and directors. Awards could be in the form of stock options, stock appreciation rights, restricted stock, restricted stock units, performance share units, and other stock-based awards. Each full-value award, defined as any award other than a stock option or stock appreciation
•Reinsurance recoverable of $6,141.5 million representing the ceded reserves related to policies on claim status (DLR cohort).