This haircut reflects the underlying risk of the specific collateral and protects our counterparty against a change in its value, but conversely subjects us to counterparty risk and limits the amount we can borrow against our investment securities. Our master repurchase agreements do not specify the haircut; rather haircuts are determined on an individual repurchase transaction basis. Throughout the three months ended March 31, 2015, haircuts on our pledged collateral remained stable and as of March 31, 2015, our weighted average haircut was less than 5% of the value of our collateral.
Under our repurchase agreements, we may be required to pledge additional assets to the repurchase agreement counterparties in the event the estimated fair value of the existing pledged collateral under such agreements declines and such counterparties demand additional collateral (a "margin call"), which may take the form of additional securities or cash. Specifically, margin calls would result from a decline in the value of our agency securities securing our repurchase agreements and prepayments on the mortgages securing such agency securities. Similarly, if the estimated fair value of our investment securities increases due to changes in interest rates or other factors, counterparties may release collateral back to us. Our repurchase agreements generally provide that the valuations for the agency MBS securing our repurchase agreements are to be obtained from a generally recognized source agreed to by the parties. However, in certain circumstances under certain of our repurchase agreements our lenders have the sole discretion to determine the value of the agency MBS securing our repurchase agreements. In such instances, our lenders are required to act in good faith in making determinations of value. Our repurchase agreements generally provide that in the event of a margin call, we must provide additional securities or cash on the same business day that a margin call is made if the lender provides us notice prior to the margin notice deadline on such day.
As of March 31, 2015, we had met all of our margin requirements and we had unrestricted cash and cash equivalents of $1.7 billion and unpledged securities of approximately $3.4 billion, including securities pledged to us, available to meet margin calls on our repurchase agreements and derivative instruments and for other corporate purposes.
Although we believe we will have adequate sources of liquidity available to us through repurchase agreement financing to execute our business strategy, there can be no assurances that repurchase agreement financing will be available to us upon the maturity of our current repurchase agreements to allow us to renew or replace our repurchase agreement financing on favorable terms or at all. If our repurchase agreement lenders default on their obligations to resell the underlying collateral back to us at the end of the term, we could incur a loss equal to the difference between the value of the collateral and the cash we originally received.
To help manage the adverse impact of interest rate changes on the value of our investment portfolio as well as our cash flows, we utilize an interest rate risk management strategy under which we use derivative financial instruments. In particular, we attempt to mitigate the risk of the cost of our variable rate liabilities increasing at a faster rate than the earnings of our long-term fixed rate assets during a period of rising interest rates. The principal derivative instruments that we use are interest rate swaps, supplemented with the use of interest rate swaptions, TBA securities, U.S. Treasury securities, U.S. Treasury futures contracts and other instruments. Please refer to Notes 3 and 6 to our consolidated financial statements in this Quarterly Report on Form 10-Q for further details regarding our use of derivative instruments.
Our derivative agreements typically require that we pledge/receive collateral on such agreements to/from our counterparties in a similar manner as we are required to under our repurchase agreements. Our counterparties, or the clearing agency in the case of centrally cleared interest rate swaps, typically have the sole discretion to determine the value of the derivative instruments and the value of the collateral securing such instruments. In the event of a margin call, we must generally provide additional collateral on the same business day.
Similar to repurchase agreements, our use of derivatives exposes us to counterparty credit risk relating to potential losses that could be recognized in the event that the counterparties to these instruments fail to perform their obligations under the contracts. We minimize this risk by limiting our counterparties to major financial institutions with acceptable credit ratings and by monitoring positions with individual counterparties.
Excluding centrally cleared interest rate swaps, as of March 31, 2015, our amount at risk with any counterparty related to our interest rate swap and swaption agreements was less than 1% of our stockholders' equity.
In the case of centrally cleared interest rate swap contracts, we could be exposed to credit risk if the central clearing agency or a clearing member defaults on its respective obligation to perform under the contract. However, we believe that the risk is minimal due to the exchange's initial and daily mark to market margin requirements and a clearinghouse guarantee fund and other resources that are available in the event of a clearing member default.