• adjusting pay and leave policies to provide additional paid time off to manage personal challenges as a result of COVID-19 including school closings and child care.
We manage our business with three reportable segments: Retail, Group and Specialty, and Healthcare Services. Beginning January 1, 2018, we exited the individual commercial fully-insured medical health insurance business, as well as certain other business in 2018, and therefore no longer report separately the Individual Commercial segment and the Other Businesses category in the current year. Previously, the Other Businesses category included businesses that were not individually reportable because they did not meet the quantitative thresholds required by generally accepted accounting principles, primarily our closed-block of commercial long-term care insurance policies which were sold in 2018. The reportable segments are based on a combination of the type of health plan customer and adjacent businesses centered on well-being solutions for our health plans and other customers, as described below. These segment groupings are consistent with information used by our Chief Executive Officer, the Chief Operating Decision Maker, to assess performance and allocate resources. See Note 18 to the consolidated financial statements included in Item 8. - Financial Statements and Supplementary Data for segment financial information.
The results of each segment are measured by income before income taxes and equity in net earnings from equity method investments, or segment earnings. Transactions between reportable segments primarily consist of sales of services rendered by our Healthcare Services segment, primarily pharmacy, provider, and clinical care services, to our Retail and Group and Specialty segment customers. Intersegment sales and expenses are recorded at fair value and eliminated in consolidation. Members served by our segments often use the same provider networks, enabling us in some instances to obtain more favorable contract terms with providers. Our segments also share indirect costs and assets. As a result, the profitability of each segment is interdependent. We allocate most operating expenses to our segments. Assets and certain corporate income and expenses are not allocated to the segments, including the portion of investment income not supporting segment operations, interest expense on corporate debt, and certain other corporate expenses. These items are managed at a corporate level. These corporate amounts are reported separately from our reportable segments and are included with intersegment eliminations.
The 2022 benchmark increase of 3.7% includes roughly 0.8% for the projected cost of COVID-19 vaccines.
Consolidated premiums increased $11.2 billion, or 17.9%, from $62.9 billion for 2019 to $74.2 billion for 2020 primarily due to higher premium revenues from Medicare Advantage and state-based contracts membership growth, higher per member Medicare Advantage premiums, and the receipt of commercial risk corridor receivables previously written off, partially offset by the impact of declining stand-alone PDP and fully-insured group commercial medical membership as more fully described in the detailed segment results discussion that follows.
Investment income was $1.2 billion for 2020, increasing $653 million, or 130.3%, from 2019, primarily due to the $745 million change in fair value of publicly-traded equity securities during 2020.
We experienced favorable medical claims reserve development related to prior fiscal years of $313 million in 2020 and $336 million in 2019. The favorable prior-period medical claims reserve development decreased the consolidated benefit ratio by approximately 40 basis points in 2020 and 50 basis points in 2019.
Interest expense was $283 million for 2020 compared to $242 million for 2019, an increase of $41 million, or 16.9%. This increase primarily was due to the higher average borrowings outstanding.
•Group Medicare Advantage membership increased 87,900 members, or 16.7%, from 525,300 members as of December 31, 2019 to 613,200 members as of December 31, 2020, primarily due to the addition of a large account in January 2020, along with net membership additions associated with the 2020 selling season. For the full year 2021, we anticipate a net membership decline in our Group Medicare Advantage offerings of approximately 50,000 members.
•Medicare stand-alone PDP membership decreased 498,500 members, or 11.4%, from 4,365,200 members as of December 31, 2019 to 3,866,700 members as of December 31, 2020, primarily resulting from terminations driven by premium and benefit adjustments experienced by members that were previously enrolled in our 2019 Humana Walmart Rx plan and the 2019 Humana Enhanced plan, which were consolidated into the Premier Rx plan in 2020. The PDP losses were partially offset by growth in the new low-price Humana Walmart Value Rx plan, driven by both new sales and plan to plan changes. For the full year 2021, we anticipate a net membership decline in our Medicare stand-alone PDP offerings of approximately 300,000 members.
Historically, our primary sources of cash have included receipts of premiums, services revenue, and investment and other income, as well as proceeds from the sale or maturity of our investment securities, and borrowings. Our primary uses of cash historically have included disbursements for claims payments, operating costs, interest on borrowings, taxes, purchases of investment securities, acquisitions, capital expenditures, repayments on borrowings, dividends, and share repurchases. Because premiums generally are collected in advance of claim payments by a period of up to several months, our business normally should produce positive cash flows during periods of increasing premiums and enrollment. Conversely, cash flows would be negatively impacted during periods of decreasing premiums and enrollment. From period to period, our cash flows may also be affected by the timing of working capital items including premiums receivable, benefits payable, and other receivables and payables. Our cash flows are impacted by the timing of payments to and receipts from CMS associated with Medicare Part D subsidies for which we do not assume risk. The use of cash flows may be limited by regulatory requirements of state departments of insurance (or comparable state regulators) which require, among other items, that our regulated subsidiaries maintain minimum levels of capital and seek approval before paying dividends from the subsidiaries to the parent. Our use of cash flows derived from our non-insurance subsidiaries, such as in our Healthcare Services segment, is generally not restricted by state departments of insurance (or comparable state regulators).
The detail of benefits payable was as follows at December 31, 2020, 2019 and 2018:
Premiums revenue is estimated by multiplying the membership covered under the various contracts by the contractual rates. Premiums revenue is recognized as income in the period members are entitled to receive services, and is net of estimated uncollectible amounts, retroactive membership adjustments, and adjustments to recognize rebates under the minimum benefit ratios required under the Health Care Reform Law. We estimate policyholder rebates by projecting calendar year minimum benefit ratios for the small group and large group markets, as defined by the Health Care Reform Law using a methodology prescribed by HHS, separately by state and legal entity. Medicare Advantage products are also subject to minimum benefit ratio requirements under the Health Care Reform Law. Estimated calendar year rebates recognized ratably during the year are revised each period to reflect current experience. Retroactive membership adjustments result from enrollment changes not yet processed, or not yet reported by an employer group or the government. We routinely monitor the collectability of specific accounts, the aging of receivables, historical retroactivity trends, estimated rebates, as well as prevailing and anticipated economic conditions, and reflect any required adjustments in current operations. Premiums received prior to the service period are recorded as unearned revenues.
All issuers of debt securities we own that were trading at an unrealized loss at December 31, 2020 remain current on all contractual payments. After taking into account these and other factors previously described, we believe these unrealized losses primarily were caused by an increase in market interest rates in the current markets since the time the debt securities were purchased. At December 31, 2020, we did not intend to sell any debt securities with an unrealized loss position in accumulated other comprehensive income, and it is not likely that we will be required to sell these debt securities before recovery of their amortized cost basis. Additionally, we did not record any material credit allowances for debt securities that were in an unrealized loss position at December 31, 2020. There were no material other-than-temporary impairments in 2019 or 2018.
Premiums revenue is estimated by multiplying the membership covered under the various contracts by the contractual rates. Premiums revenue is recognized as income in the period members are entitled to receive services, and is net of estimated uncollectible amounts, retroactive membership adjustments, and adjustments to recognize rebates under the minimum benefit ratios required under the Health Care Reform Law. We estimate policyholder
Benefits expense is recognized in the period in which services are provided and includes an estimate of the cost of services which have been incurred but not yet reported, or IBNR. Our reserving practice is to consistently recognize the actuarial best point estimate within a level of confidence required by actuarial standards. Actuarial standards of practice generally require a level of confidence such that the liabilities established for IBNR have a greater probability of being adequate versus being insufficient, or such that the liabilities established for IBNR are sufficient to cover obligations under an assumption of moderately adverse conditions. Adverse conditions are situations in which the actual claims are expected to be higher than the otherwise estimated value of such claims at the time of the estimate. Therefore, in many situations, the claim amounts ultimately settled will be less than the estimate that satisfies the actuarial standards of practice.
In June 2016, the FASB issued guidance introducing a new model for recognizing credit losses on financial instruments based on an estimate of current expected credit losses. The guidance was effective for us beginning January 1, 2020. The new current expected credit losses (CECL) model generally calls for the immediate recognition of all expected credit losses and applies to loans, accounts and trade receivables as well as other financial assets measured at amortized cost, loan commitments and off-balance sheet credit exposures, debt securities and other financial assets measured at fair value through other comprehensive income, and beneficial interests in securitized financial assets. The new guidance replaces the current incurred loss model for measuring expected credit losses, requires expected losses on available for sale debt securities to be recognized through an allowance for credit losses rather than as reductions in the amortized cost of the securities, and provides for additional disclosure requirements. Our investment portfolio consists primarily of available for sale debt securities. We adopted the new standard effective January 1, 2020. Due to the high concentration of our financial assets measured at amortized cost being with the federal government resulting in zero nonpayment risk as well as our available for sale debt securities primarily being in an unrealized gain position, the adoption of the new standard did not have a material impact on our results of operations, financial condition, or cash flows.
Investment securities classified as current and long-term were as follows at December 31, 2020 and 2019, respectively:
Gross gains and gross losses on investment securities include both the gain resulting from the initial conversion of our prior ownership interest in certain privately held companies into common stock upon such companies' initial public offering, or IPO, and subsequent changes in the market value of such securities from the IPO through December 31, 2020, which combined to total $837 million and $91 million, respectively.
Changes in the carrying amount of goodwill for our reportable segments for the years ended December 31, 2020 and 2019 were as follows:
The following tables provide information about incurred and paid claims development for the Retail segment as of December 31, 2020, net of reinsurance.
At December 31, 2020, benefits payable for our Group and Specialty segment included IBNR of approximately $594 million, primarily associated with claims incurred in 2020. The cumulative number of reported claims as of December 31, 2020 was approximately 8.6 million for claims incurred in 2020, 10.0 million for claims incurred in 2019, and 10.9 million for claims incurred in 2018.
In connection with employee stock plans, we acquired 0.2 million common shares for $70 million in 2020, 0.2 million common shares for $70 million in 2019, and 0.4 million common shares for $116 million in 2018.