If the third condition exists, the OTTI is separated into (i) the amount relating to credit loss (the "credit component") and (ii) the amount relating to all other factors (the "non-credit components"). Only the credit component is recognized in earnings, with the non-credit components recognized in OCI. However, in evaluating if the third condition exists, our investments in agency securities typically would not have a credit component since the principal and interest are guaranteed by a GSE and, therefore, any unrealized loss is not the result of a credit loss. In addition, since we designate our agency securities as available-for-sale securities with unrealized gains and losses recognized in OCI, any impairment loss for non-credit components is already recognized in OCI.
We did not recognize any OTTI charges on our investment securities for fiscal years 2014, 2013 or 2012.
Interest income is accrued based on the outstanding principal amount of the investment securities and their contractual terms. Premiums or discounts associated with the purchase of investment securities are amortized or accreted into interest income, respectively, over the projected lives of the securities, including contractual payments and estimated prepayments using the effective interest method in accordance with ASC Subtopic 310-20, Receivables—Nonrefundable Fees and Other Costs ("ASC 310-20").
We estimate long-term prepayment speeds of our agency securities using a third-party service and market data. The third-party service estimates prepayment speeds using models that incorporate the forward yield curve, current mortgage rates and mortgage rates of the outstanding loans, age and size of the outstanding loans, loan-to-value ratios, volatility and other factors. We review the prepayment speeds estimated by the third-party service and compare the results to market consensus prepayment speeds, if available. We also consider historical prepayment speeds and current market conditions to validate the reasonableness of the prepayment speeds estimated by the third-party service and, based on our Manager's judgment, we may make adjustments to its estimates. Actual and anticipated prepayment experience is reviewed quarterly and effective yields are recalculated when differences arise between (i) our previously estimated future prepayments and (ii) the actual prepayments to date plus our currently estimated future prepayments. If the actual and estimated future prepayment experience differs from our prior estimate of prepayments, we are required to record an adjustment in the current period to the amortization or accretion of premiums and discounts for the cumulative difference in the effective yield through the reporting date.
We finance the acquisition of securities for our investment portfolio through repurchase transactions under master repurchase agreements. Pursuant to ASC Topic 860, Transfers and Servicing ("ASC 860"), we account for repurchase transactions as collateralized financing transactions, which are carried at their contractual amounts (cost), plus accrued interest, as specified in the respective transactions. Our repurchase agreements typically have maturities of less than one year, but may extend up to five years or more. Interest rates under our repurchase agreements generally correspond to one, three or six month LIBOR plus or minus a fixed spread. The fair value of our repurchase agreements is assumed to equal cost as the interest rates are considered to be at market.
Reverse Repurchase Agreements and Obligation to Return Securities Borrowed under Reverse Repurchase Agreements
We from time to time borrow securities to cover short sales of U.S. Treasury securities through reverse repurchase transactions under our master repurchase agreements (see Derivatives Instruments below). We account for these as securities borrowing transactions and recognize an obligation to return the borrowed securities at fair value on the balance sheet based on the value of the underlying borrowed securities as of the reporting date. Our reverse repurchase agreements generally mature daily. The fair value of our reverse repurchase agreements is assumed to equal cost as the interest rates are generally reset daily.
We use a variety of derivative instruments to hedge a portion of our exposure to market risks, including interest rate risk, prepayment risk and extension risk. The objective of our risk management strategy is to reduce fluctuations in net book value over a range of interest rate scenarios. In particular, we attempt to mitigate the risk of the cost of our variable rate liabilities increasing during a period of rising interest rates. The principal instruments that we use are interest rate swaps and options to enter into interest rate swaps ("swaptions"). We also utilize forward contracts for the purchase or sale of agency MBS securities on a generic pool basis, or a TBA contract, and we utilize U.S. Treasury securities and U.S. Treasury futures contracts, primarily through short sales. We may also purchase or write put or call options on TBA securities and we may invest in other types of mortgage derivatives, such as interest and principal-only securities.
We may also enter into TBA contracts as a means of investing in and financing agency securities (thereby increasing our "at risk" leverage) or as a means of disposing of or reducing our exposure to agency securities (thereby reducing our "at risk" leverage). Pursuant to TBA contracts, we agree to purchase or sell, for future delivery, agency securities with certain principal and interest