interest rate cash flow hedges 2013 we report changes in the fair value of cash flow hedges in accumulated other comprehensive loss until the hedged item affects earnings . at both december 31 , 2008 and 2007 , we had reductions of $ 4 million recorded as an accumulated other comprehensive loss that is being amortized on a straight-line basis through september 30 , 2014 . as of december 31 , 2008 and 2007 , we had no interest rate cash flow hedges outstanding . earnings impact 2013 our use of derivative financial instruments had the following impact on pre-tax income for the years ended december 31 : millions of dollars 2008 2007 2006 .
what was the difference in billions of sold receivables from 2007 to 2008?
fair value of debt instruments 2013 the fair value of our short- and long-term debt was estimated using quoted market prices , where available , or current borrowing rates . at december 31 , 2008 , the fair value of total debt is approximately $ 247 million less than the carrying value . at december 31 , 2007 , the fair value of total debt exceeded the carrying value by approximately $ 96 million . at december 31 , 2008 and 2007 , approximately $ 320 million and $ 181 million , respectively , of fixed-rate debt securities contained call provisions that allowed us to retire the debt instruments prior to final maturity , with the payment of fixed call premiums , or in certain cases , at par . sale of receivables 2013 the railroad transfers most of its accounts receivable to union pacific receivables , inc . ( upri ) , a bankruptcy-remote subsidiary , as part of a sale of receivables facility . upri sells , without recourse on a 364-day revolving basis , an undivided interest in such accounts receivable to investors . the total capacity to sell undivided interests to investors under the facility was $ 700 million and $ 600 million at december 31 , 2008 and 2007 , respectively . the value of the outstanding undivided interest held by investors under the facility was $ 584 million and $ 600 million at december 31 , 2008 and 2007 , respectively . upri reduced the outstanding undivided interest held by investors due to a decrease in available receivables at december 31 , 2008 . the value of the outstanding undivided interest held by investors is not included in our consolidated financial statements . the value of the undivided interest held by investors was supported by $ 1015 million and $ 1071 million of accounts receivable held by upri at december 31 , 2008 and 2007 , respectively . at december 31 , 2008 and 2007 , the value of the interest retained by upri was $ 431 million and $ 471 million , respectively . this retained interest is included in accounts receivable in our consolidated financial statements . the interest sold to investors is sold at carrying value , which approximates fair value , and there is no gain or loss recognized from the transaction . the value of the outstanding undivided interest held by investors could fluctuate based upon the availability of eligible receivables and is directly affected by changing business volumes and credit risks , including default and dilution . if default or dilution percentages were to increase one percentage point , the amount of eligible receivables would decrease by $ 6 million . should our credit rating fall below investment grade , the value of the outstanding undivided interest held by investors would be reduced , and , in certain cases , the investors would have the right to discontinue the facility . the railroad services the sold receivables ; however , the railroad does not recognize any servicing asset or liability as the servicing fees adequately compensate us for these responsibilities . the railroad collected approximately $ 17.8 billion and $ 16.1 billion during the years ended december 31 , 2008 and 2007 , respectively . upri used certain of these proceeds to purchase new receivables under the facility. .