gains and losses will differ from those estimated because of changes or differences in market rates and interrelationships, hedging instruments and hedge percentages, timing and other factors. 

The estimated maximum one-day loss in fair value on our foreign currency sensitive derivative financial instruments, derived using the VaR model, was $34 million and $21 million at May 31, 2013 and 2012, respectively. The VaR increased year-over-year as a result of an increase in the total notional value of our foreign currency derivative portfolio combined with a longer average duration on our outstanding trades at May 31, 2013. Such a hypothetical loss in the fair value of our derivatives would be offset by increases in the value of the underlying transactions being hedged. The average monthly change in the fair values of foreign currency forward and foreign currency option derivative instruments was $49 million and $87 million during fiscal 2013 and fiscal 2012, respectively. 

The instruments not included in the VaR are intercompany loans denominated in non-functional currencies, fixed interest rate Japanese Yen denominated debt, fixed interest rate U.S. Dollar denominated debt and interest rate swaps. Intercompany loans and related interest amounts are eliminated in consolidation. Furthermore, our non-functional currency intercompany loans are substantially hedged against foreign exchange risk through the use of forward contracts, which are included in the VaR calculation above. Therefore, we consider the interest rate and foreign currency market risks associated with our non-functional currency intercompany loans to be immaterial to our consolidated financial position, results from operations and cash flows. 

Details of third-party debt and interest rate swaps are provided in the table below. The table presents principal cash flows and related weighted average interest rates by expected maturity dates. Weighted average interest rates for the fixed rate swapped to floating rate debt reflect the effective interest rates at May 31, 2013.