U.S. debt holdings: $2.67 trillion
The largest portion of U.S. Intergovernmental Securities and the biggest holder of U.S. debt is the Federal Old-Age and Survivors Insurance Trust Fundand the Federal Disability Insurance Trust Fund,together comprising the Social Security Trust Fund or OASDI. When tax revenues for the Social Security exceed current expenses, the funds are legally obligated to invest in interest-bearing Federal securities guaranteed by the government, such as bonds from the U.S. Treasury.
According to the Treasury, as of December 2011, the Federal Old-Age and Survivors Insurance Trust Fund held $2.54 trillion of U.S. debt, while the Federal Disability Insurance Trust Fund held $153.9 billion, together more than doubling the amount of debt held by the Federal Reserve or China. Although the Social Security Trust Fund commands huge debt holdings in comparison to others, the amount of debt has held relatively stable, increasing by only $173 billion - approximately 7% - since 2009.