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 86% ( 86 % ) and 94% ( 94 % ) as of december 31 , 2018 and 2017 , 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 .
in 2018 what was the ratio of the impact to the fair market value of the 10% ( 10 % ) increase in interest rates to the 10% ( 10 % ) decrease in interest rates 3 2018
we have used interest rate swaps for risk management purposes to manage our exposure to changes in interest rates . we did not have any interest rate swaps outstanding as of december 31 , 2018 . we had $ 673.5 of cash , cash equivalents and marketable securities as of december 31 , 2018 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 2018 and 2017 , we had interest income of $ 21.8 and $ 19.4 , respectively . based on our 2018 results , a 100 basis-point increase or decrease in interest rates would affect our interest income by approximately $ 6.7 , assuming that all cash , cash equivalents and marketable securities are impacted in the same manner and balances remain constant from year-end 2018 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 foreign currencies that most favorably impacted our results during the year ended december 31 , 2018 were the euro and british pound sterling . the foreign currencies that most adversely impacted our results during the year ended december 31 , of 2018 were the argentine peso and brazilian real . based on 2018 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 2018 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 .