Off-budget/on-budget? Few things make less sense than the federal finances. Here is some official info ... 322 pages.
http://www.whitehouse.gov/omb/budget/fy2006/pdf/hist.pdf"Social Security and the Postal Service are now off-budget, they continue to be Federal programs. Indeed, Social Security currently accounts for about one-fourth of all Federal receipts and over one-fifth of all Federal spending. Hence, the budget documents include these funds and focus on the Federal totals that combine the on-budget and off-budget amounts. Various budget tables and charts show total Federal receipts, outlays, and surpluses and deficits, and divide these totals between the portions that are on-budget and off-budget.