From "A Citizen's Guide to the 2012 Financial Report of the U.S. Government"

http://www.fms.treas.gov/frsummary/frsummary2012.pdf

"...the largest contributors to the Government’s net cost in FY 2012, as is the case in most years, include the Departments of Health and Human Services (HHS) and Defense (DoD) and the Social Security Administration (SSA). The bulk of HHS and SSA costs come from major social insurance and postemployment benefits programs administered by those agencies (e.g., Medicare for HHS, and Social Security for SSA)."

"It is estimated that preventing the debt-to-GDP ratio from rising over the next 75 years would require some combination of expenditure reductions and revenue increases that amount, on average, to 2.7 percent of GDP over each of the next 75 years. The timing of changes to noninterest spending and receipts that close this “75-year fiscal gap” has important implications for the well-being of future generations."

http://www.fms.treas.gov/frsummary/frsummary2012.pdf