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 level of Agency securities eligible for TBA delivery enhances our liquidity profile and provides price support for our TBA eligible securities at or above generic TBA prices. As of December 31, 2017, approximately 90% of our fixed rate Agency RMBS portfolio was eligible for TBA delivery.
To the extent we raise additional equity capital we may use cash proceeds from such transactions to purchase additional investment securities, make scheduled payments of principal and interest on our funding liabilities and/or for other general corporate purposes. There can be no assurance, however, that we will be able to raise additional equity capital at any particular time or on any particular terms. Furthermore, when the trading price of our common stock is significantly less than our estimate of our current tangible net book value per common share, among other conditions, we may repurchase shares of our common stock, subject to the provisions of a stock repurchase program in effect at such time.
OFF-BALANCE SHEET ARRANGEMENTS
As of December 31, 2017, we did not maintain relationships with unconsolidated entities or financial partnerships, such as entities often referred to as structured finance, or special purpose or variable interest entities, established to facilitate off-balance sheet arrangements or other contractually narrow or limited purposes. Additionally, as of December 31, 2017, we had not guaranteed obligations of unconsolidated entities or entered into a commitment or intent to provide funding to such entities.
AGGREGATE CONTRACTUAL OBLIGATIONS
The following table summarizes the effect on our liquidity and cash flows from contractual obligations for repurchase agreements and related interest expense (in millions):
Interest expense 1
Interest expense is calculated based on the weighted average interest rates on our repurchase agreements as of December 31, 2017.
This document contains forward looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These forward looking statements are based on estimates, projections, beliefs and assumptions of our management as of the date of this Annual Report on Form 10-K and involve risks and uncertainties in predicting future results and conditions. Our actual performance could differ materially from those projected or anticipated in any forward looking statements due to a variety of factors, including, without limitation, changes in interest rates, the yield curve or prepayment rates; the availability and terms of financing; changes in the market value of our assets; the effectiveness of our risk mitigation strategies; conditions in the market for Agency and other mortgage securities; or legislative or regulatory changes that affect our status as a REIT or our exemption from the Investment Company Act of 1940 or that affect the GSE’s or secondary mortgage market in which we participate. A discussion of risks and uncertainties that could cause actual results to differ from any of our forward looking statements is included in this document under Item 1A. Risk Factors. We caution readers not to place undue reliance on our forward looking statements.