Note 10. Stockholders' Equity
Pursuant to our amended and restated certificate of incorporation, we are authorized to designate and issue up to 10.0 million shares of preferred stock in one or more classes or series. As of December 31, 2017 and June 30, 2018, 8,050 shares were designated as 7.750% Series B Cumulative Redeemable Preferred Stock ("Series B Preferred Stock") and 13,800 shares were designated as 7.00% Series C Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock ("Series C Preferred Stock"). Shares of Series B and Series C Preferred Stock are represented by depositary shares equal to 1/1000 interest in each share of Series B and Series C Preferred Stock, respectively. As of December 31, 2017 and June 30, 2018, we had 7,000 shares of Series B Preferred Stock and 13,000 shares of Series C Preferred Stock outstanding (represented by 7.0 million Series B depositary shares and 13.0 million Series C depositary shares outstanding, respectively) and 9,980,000 of authorized but unissued shares of preferred stock.
Holders of depository shares underlying our Series B Preferred Stock are entitled to receive cumulative cash dividends at a rate of 7.750% per annum of their $25.00 per depositary share liquidation preference. Holders of depositary shares underlying our Series C Preferred Stock are entitled to receive cumulative cash dividends at a rate of 7.00% per annum up to, and including, October 14, 2022 and thereafter at a floating rate equal to three-month LIBOR plus a spread of 5.111% per annum of their $25.00 per depositary share liquidation preference. Dividends are payable quarterly in arrears on the 15th day of each January, April, July and October. As of June 30, 2018, we had declared all required quarterly dividends on our preferred stock.
Our preferred stock ranks senior to our common stock with respect to the payment of dividends and the distribution of assets upon a voluntary or involuntary liquidation, dissolution or winding up of the Company. Our preferred stock has no stated maturity, is not subject to any sinking fund or mandatory redemption and ranks on parity with each other. Under certain circumstances upon a change of control, our preferred stock is convertible to shares of our common stock. Holders of our preferred stock and depository shares underlying our preferred stock have no voting rights, except under limited conditions. Beginning on May 8, 2019 and October 15, 2022, depository shares underlying our Series B and Series C Preferred Stock, respectively, will be redeemable at $25.00 per depositary share, plus accumulated and unpaid dividends (whether or not declared) exclusively at our option. We may redeem shares of our preferred stock prior to our optional redemption date under certain circumstances intended to preserve our qualification as a REIT for Federal income tax purposes.
Common Stock Offerings
In May 2018, we completed a public offering in which 34.5 million shares of our common stock were sold to the underwriters for proceeds of $633 million, or $18.35 per common share, net of offering costs. In May 2017, we completed a public offering in which 24.5 million shares of our common stock were sold to the underwriters for proceeds of $503 million, or $20.51 per common share, net of offering costs.
At-the-Market Offering Program
We have entered into agreements with sales agents to publicly offer and sell shares of our common stock in privately negotiated and/or at-the-market transactions from time-to-time up to a maximum aggregate offering price of our common stock. On June 14, 2018, our existing sales agreement terminated and we entered into new sales agreements for shares of our common stock having an aggregate offering price of up to $1 billion. During the three and six months ended June 30, 2018, we sold 8.3 million shares of our common stock under sales agreements for proceeds of $155 million, or $18.73 per common share, net of offering costs. During the three and six months ended June 30, 2017, we did not sell shares of our common stock under the sales agreements. As of June 30, 2018, shares of our common stock with an aggregate offering price of $881 million remained available for issuance under this program.