swaptions, U.S. Treasury securities, U.S. Treasury futures contracts and TBA securities. Please refer to Notes 3 and 6 to our Consolidated Financial Statements in this Form 10-Q for further details regarding our use of derivative instruments.
Our derivative agreements typically require that we pledge / receive collateral in a similar manner as we are required to under our repurchase agreements. Our counterparties, or the central clearing agency, 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 provide additional collateral generally on the same or next business day. We minimize counterparty credit risk associated with our derivative instruments by limiting our counterparties to central clearing exchanges and major financial institutions with acceptable credit ratings and by monitoring positions with individual counterparties. Excluding centrally cleared derivative instruments, as of September 30, 2017, 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 derivative instruments, 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 exchanges' initial and daily mark to market margin requirements and clearinghouse guarantee funds and other resources that are available in the event of a clearing member default.
TBA Dollar Roll Transactions
TBA dollar roll transactions represent a form of off-balance sheet financing accounted for as derivative instruments and we may use them as a means of leveraging (or deleveraging) our investment portfolio through the use of long (or short) TBA contracts (see Notes 3 and 6 to our Consolidated Financial Statements in this Form 10-Q).
Under certain market conditions, it may be uneconomical for us to roll our TBA contracts into future months and we may need to take or make physical delivery of the underlying securities. If we were required to take physical delivery to settle a long TBA contract, we would have to fund our total purchase commitment with cash or other financing sources and our liquidity position could be negatively impacted. As of September 30, 2017, we had a net long TBA position with a total market value and a total cost basis of $19.4 billion and a net carrying value of $(24) million recognized in derivative assets/(liabilities), at fair value, on our Consolidated Balance Sheets in this Form 10-Q.
Our TBA dollar roll contracts are also subject to margin requirements governed by the Mortgage-Backed Securities Division ("MBSD") of the FICC and by our prime brokerage agreements, which may establish margin levels in excess of the MBSD. Such provisions require that we establish an initial margin based on the notional value of the TBA contract, which is subject to increase if the estimated fair value of our TBA contract or the estimated fair value of our pledged collateral declines. The MBSD has the sole discretion to determine the value of our TBA contracts and of the pledged collateral securing such contracts. In the event of a margin call, we must generally provide additional collateral on the same business day.
Settlement of our TBA obligations by taking delivery of the underlying securities as well as satisfying margin requirements could negatively impact our liquidity position. However, since we do not use TBA dollar roll transactions as our primary source of financing, we believe that we will have adequate sources of liquidity to meet such obligations.
Federal Home Loan Bank Advances
In February 2017, our wholly-owned captive insurance subsidiary's membership to the FHLB of Des Moines was terminated pursuant to the FHFA's final rule on changes to regulations concerning FHLB membership criteria released in January 2016. All of our outstanding FHLB advances were repaid prior to termination.
Bethesda Securities Regulatory Capital Requirements
Bethesda Securities is subject to regulations of the securities business that include but are not limited to trade practices, capital structure, recordkeeping and conduct of directors, officers and employees. As a self-clearing registered broker-dealer, Bethesda Securities is required to maintain minimum net regulatory capital as defined by SEC Rule 15c3-1 (the "Rule"). As of September 30, 2017, the minimum net capital required was $0.3 million and Bethesda Securities had excess net capital of $204.0 million. Regulatory capital in excess of the minimum required by the Rule is held to meet levels required by clearing organizations, the clearing bank and other repo counterparties.
Asset Sales and TBA Eligible Securities
We maintain a portfolio of highly liquid mortgage-backed securities. We may sell our Agency securities through the TBA market by delivering them into TBA contracts, subject to "good delivery" provisions promulgated by the Securities Industry and Financial Markets Association ("SIFMA"). We may alternatively sell Agency securities that have more unique attributes on a specified basis when such securities trade at a premium over generic TBA securities or if the securities are not otherwise eligible for TBA delivery. Since the TBA market is the second most liquid market (after the U.S. Treasury market), maintaining a significant