.5 billion at december 31 , 2002 . the commercial paper issued is backed by sufficient collateral , credit enhance- ments and commitments to provide liquidity to support receiving at least an a-1 , p-1 and , in certain cases , an f1 rating . the firm had commitments to provide liquidity on an asset- specific basis to these vehicles in an amount up to $ 18.0 billion at december 31 , 2003 , and $ 23.5 billion at december 31 , 2002