"The Social Security program pays benefits to retired or disabled workers and their family members and to the family members of deceased workers. As of July 2017, there were 61.5 million Social Security beneficiaries. Approximately 68% of those beneficiaries were retired workers and 14% were disabled workers. The remaining beneficiaries were the survivors of deceased insured workers or the spouses and children of retired or disabled workers. Social Security is financed primarily by payroll taxes paid by covered workers and their employers. The program is also credited with federal income taxes paid by some beneficiaries on a portion of their benefits, reimbursements from the General Fund of the Treasury for various purposes, and interest income from investments held by the Social Security trust funds. Social Security tax revenues are invested in U.S. government securities (special issues) held by the trust funds, and these securities earn interest. The tax revenues exchanged for the U.S. government securities are deposited into the General Fund of the Treasury and are indistinguishable from revenues in the General Fund that come from other sources. Because the assets held by the trust funds are U.S. government securities, the trust fund balance represents the amount of money owed to the Social Security trust funds by the General Fund of the Treasury. Funds needed to pay Social Security benefits and administrative expenses come from the redemption or sale of U.S. government securities held by the trust funds."
CRS Report for Congress, RL33028
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html