We expect to continue managing the pacing of the remaining $373.1 million (as of October 24, 2017) under the 2011 Buyback in response to general market conditions and other relevant factors. We expect to fund further repurchases of our common stock through a combination of cash on hand, cash generated by operations and borrowings under our credit facilities. Purchases under the 2011 Buyback are subject to us having available cash to fund repurchases. For more information on the 2011 Buyback, see Part II, Item 2 in this Quarterly Report on Form 10-Q.
Sales of Equity Securities. We receive proceeds from sales of our equity securities pursuant to our employee stock purchase plan (“ESPP”) and upon exercise of stock options granted under our equity incentive plans. For the nine months ended September 30, 2017, we received an aggregate of $105.7 million in proceeds upon exercises of stock options and sales pursuant to the ESPP.
Distributions. As a REIT, we must annually distribute to our stockholders an amount equal to at least 90% of our REIT taxable income (determined before the deduction for distributed earnings and excluding any net capital gain). Generally, we have distributed, and expect to continue to distribute, all or substantially all of our REIT taxable income after taking into consideration our utilization of NOLs. We have distributed an aggregate of approximately $4.0 billion to our common stockholders, including the dividend paid in October 2017, primarily subject to taxation as ordinary income.
The amount, timing and frequency of future distributions will be at the sole discretion of our Board of Directors and will depend on various factors, a number of which may be beyond our control, including our financial condition and operating cash flows, the amount required to maintain our qualification for taxation as a REIT and reduce any income and excise taxes that we otherwise would be required to pay, limitations on distributions in our existing and future debt and preferred equity instruments, our ability to utilize NOLs to offset our distribution requirements, limitations on our ability to fund distributions using cash generated through our taxable REIT subsidiaries and other factors that our Board of Directors may deem relevant.
We have one series of preferred stock outstanding, our 5.50% Mandatory Convertible Preferred Stock, Series B, par value $0.01 per share (the “Series B”), with a dividend rate of 5.50%. Dividends are payable quarterly in arrears, subject to declaration by our Board of Directors. During the nine months ended September 30, 2017, pursuant to the terms of the Certificate of Designations governing our 5.25% Mandatory Convertible Preferred Stock, Series A, par value $0.01 per share (the “Series A”), all outstanding shares of the Series A converted into shares of our common stock at a conversion rate of 0.9337 per share.
During the nine months ended September 30, 2017, we paid dividends of $2.625 per share, or $15.8 million, to Series A preferred stockholders of record and $41.25 per share, or $56.7 million, to Series B preferred stockholders of record.
In addition, in October 2017, we declared a dividend of $13.75 per share, or $18.9 million, payable on November 15, 2017 to Series B preferred stockholders of record at the close of business on November 1, 2017.
During the nine months ended September 30, 2017, we paid $1.84 per share, or $786.7 million, to common stockholders of record. In addition, we declared a distribution of $0.66 per share, or $283.3 million, paid on October 17, 2017 to our common stockholders of record at the close of business on September 29, 2017.
We accrue distributions on unvested restricted stock units, which are payable upon vesting. As of September 30, 2017, the amount accrued for distributions payable related to unvested restricted stock units was $8.0 million. During the nine months ended September 30, 2017, we paid $2.9 million of distributions upon the vesting of restricted stock units.
Factors Affecting Sources of Liquidity
As discussed in the “Liquidity and Capital Resources” section of the 2016 Form 10-K, our liquidity depends on our ability to generate cash flow from operating activities, borrow funds under our credit facilities and maintain compliance with the contractual agreements governing our indebtedness. We believe that the debt agreements discussed below represent our material debt agreements that contain covenants, our compliance with which would be material to an investor’s understanding of our financial results and the impact of those results on our liquidity.