item 7a . quantitative and qualitative disclosures about market risk ( amounts in millions ) in the normal course of business , we are exposed to market risks related to interest rates , foreign currency rates and certain balance sheet items . from time to time , we use derivative instruments , pursuant to established guidelines and policies , to manage some portion of these risks . derivative instruments utilized in our hedging activities are viewed as risk management tools and are not used for trading or speculative purposes . interest rates our exposure to market risk for changes in interest rates relates primarily to the fair market value and cash flows of our debt obligations . the majority of our debt ( approximately 89% ( 89 % ) and 91% ( 91 % ) as of december 31 , 2015 and 2014 , respectively ) bears interest at fixed rates . we do have debt with variable interest rates , but a 10% ( 10 % ) increase or decrease in interest rates would not be material to our interest expense or cash flows . the fair market value of our debt is sensitive to changes in interest rates , and the impact of a 10% ( 10 % ) change in interest rates is summarized below . increase/ ( decrease ) in fair market value as of december 31 , 10% ( 10 % ) increase in interest rates 10% ( 10 % ) decrease in interest rates .
what is the total difference , in millions , in 2015 between a 10% ( 10 % ) increase and 10% ( 10 % ) decrease in interest rates?
we have used interest rate swaps for risk management purposes to manage our exposure to changes in interest rates . we do not have any interest rate swaps outstanding as of december 31 , 2015 . we had $ 1509.7 of cash , cash equivalents and marketable securities as of december 31 , 2015 that we generally invest in conservative , short-term bank deposits or securities . the interest income generated from these investments is subject to both domestic and foreign interest rate movements . during 2015 and 2014 , we had interest income of $ 22.8 and $ 27.4 , respectively . based on our 2015 results , a 100-basis-point increase or decrease in interest rates would affect our interest income by approximately $ 15.0 , assuming that all cash , cash equivalents and marketable securities are impacted in the same manner and balances remain constant from year-end 2015 levels . foreign currency rates we are subject to translation and transaction risks related to changes in foreign currency exchange rates . since we report revenues and expenses in u.s . dollars , changes in exchange rates may either positively or negatively affect our consolidated revenues and expenses ( as expressed in u.s . dollars ) from foreign operations . the primary foreign currencies that impacted our results during 2015 included the australian dollar , brazilian real , british pound sterling and euro . based on 2015 exchange rates and operating results , if the u.s . dollar were to strengthen or weaken by 10% ( 10 % ) , we currently estimate operating income would decrease or increase approximately 4% ( 4 % ) , assuming that all currencies are impacted in the same manner and our international revenue and expenses remain constant at 2015 levels . the functional currency of our foreign operations is generally their respective local currency . assets and liabilities are translated at the exchange rates in effect at the balance sheet date , and revenues and expenses are translated at the average exchange rates during the period presented . the resulting translation adjustments are recorded as a component of accumulated other comprehensive loss , net of tax , in the stockholders 2019 equity section of our consolidated balance sheets . our foreign subsidiaries generally collect revenues and pay expenses in their functional currency , mitigating transaction risk . however , certain subsidiaries may enter into transactions in currencies other than their functional currency . assets and liabilities denominated in currencies other than the functional currency are susceptible to movements in foreign currency until final settlement . currency transaction gains or losses primarily arising from transactions in currencies other than the functional currency are included in office and general expenses . we regularly review our foreign exchange exposures that may have a material impact on our business and from time to time use foreign currency forward exchange contracts or other derivative financial instruments to hedge the effects of potential adverse fluctuations in foreign currency exchange rates arising from these exposures . we do not enter into foreign exchange contracts or other derivatives for speculative purposes. .