(In millions) Years ended June 30, 2019 2018 2017 Average investment balances at cost: Corporate investments $ 4,817.3 $ 5,112.4 $ 6,143.3 Funds held for clients 25,458.5 24,332.6 23,023.5 Total $ 30,275.8 $ 29,445.0 $ 29,166.8 Average interest rates earned exclusive of realized (gains)/losses on: Corporate investments 2.0 % 1.6 % 1.2 % Funds held for clients 2.2% 1.9% 1.7% Total 2.2 % 1.9 % 1.6 % Realized gains on available-for-sale securities $ (1.8 ) $ (2.0 ) $ (5.3 ) Realized losses on available-for-sale securities 2.7 4.5 3.1 Net realized losses/(gains) on available-for-sale securities $ 0.9 $ 2.5 $ (2.2 ) As of June 30: Net unrealized pre-tax gains/(losses) on available-for-sale securities $ 287.5 $ (355.7 ) $ 102.5 Total available-for-sale securities at fair value $ 24,859.1 $ 22,776.2 $ 21,901.1 We are exposed to interest rate risk in relation to securities that mature, as the proceeds from maturing securities are reinvested. Factors that influence the earnings impact of interest rate changes include, among others, the amount of invested funds and the overall portfolio mix between short-term and long-term investments. This mix varies during the fiscal year and is impacted by daily interest rate changes. The annualized interest rate earned on our entire portfolio increased from 1.9% for fiscal 2018 to 2.2% for fiscal 2019. A hypothetical change in both short-term interest rates (e.g., overnight interest rates or the federal funds rate) and intermediate-term interest rates of 25 basis points applied to the estimated average investment balances and any related short-term borrowings would result in approximately a $13 million impact to earnings before income taxes over the ensuing twelve-month period ending June 30, 2020. A hypothetical change in only short-term interest rates of 25 basis points applied to the estimated average short-term investment balances and any related short-term borrowings would result in approximately a $5 million impact to earnings before income taxes over the ensuing twelve-month period ending June 30, 2020. We are exposed to credit risk in connection with our available-for-sale securities through the possible inability of the borrowers to meet the terms of the securities. We limit credit risk by investing in investment-grade securities, primarily AAA and AA- rated securities, as rated by Moody’s, Standard & Poor’s, DBRS for Canadian dollar denominated securities, and Fitch for asset-backed and commercial-mortgage-backed securities. Approximately 79% of our available-for-sale securities held a AAA or AA rating at June 30, 2019. In addition, we limit amounts that can be invested in any security other than U.S. government and government agency, Canadian government and United Kingdom Gilt securities. We operate and transact business in various foreign jurisdictions and are therefore exposed to market risk from changes in foreign currency exchange rates that could impact our consolidated results of operations, financial position, or cash flows. We manage our exposure to these market risks through our regular operating and financing activities and, when deemed appropriate, through the use of derivative financial instruments. We may use derivative financial instruments as risk management tools and not for trading purposes. We had no derivative financial instruments outstanding at June 30, 2019 or 2018. 40 Details regarding our overall investment portfolio are as follows: