Stockholders' Equity	12 Months Ended
Dec. 31, 2017
Equity [Abstract]
Stockholders' Equity	Note 11. Stockholders’ Equity Sales Agreement On May 5, 2016, the Company entered into Amendment No. 2 to the Controlled Equity Offering^SM Sales Agreement, dated August 31, 2012, as previously amended on March 21, 2014, (together, the “Prior Cantor Agreement”) with Cantor Fitzgerald & Co. (“Cantor”) that provides for the issuance and sale of shares of the Company’s common stock having an aggregate offering price of up to $132.2 million through Cantor over the term of the Amended Cantor Agreement. As a result of Amendment No. 2, at May 5, 2016, the Company had $70 million of common stock available to be sold under the Amended Cantor Agreement. During the years ended December 31, 2017 and 2016, 11.0 million and 3.5 million shares, respectively, of the Company’s common stock were sold under the Prior Cantor Agreement for net proceeds of $30.3 million and $22.0 million, respectively. On August 4, 2017, the Company entered into Amendment No. 3 to the Prior Cantor Agreement (together, the “Amended Cantor Agreement”). The Amended Cantor Agreement became effective on January 8, 2018, and provides for the issuance and sale of shares of the Company’s common stock having an aggregate offering price of up to $70.0 million through Cantor, which amount includes the $31.4 million of unsold shares of common stock available for sale under the Prior Cantor Agreement immediately prior to the effectiveness of the Amended Cantor Agreement. Under the Amended Cantor Agreement, Cantor also acts as the Company’s sales agent and receives compensation based on an aggregate of 2% of the gross proceeds on the sale price per share of its common stock. The issuance and sale of these shares by the Company pursuant to the Amended Cantor Agreement are deemed an “at-the-market” offering and are registered under the Securities Act of 1933, as amended. Stockholder Rights Plan In October 2009, the Company’s Board of Directors adopted an amendment to its 1999 stockholder rights plan, commonly referred to as a “poison pill,” to reduce the exercise price, extend the expiration date and revise certain definitions under the plan. The stockholder rights plan is intended to deter hostile or coercive attempts to acquire the Company. The stockholder rights plan enables stockholders to acquire shares of the Company’s common stock, or the common stock of an acquirer, at a substantial discount to the public market price should any person or group acquire more than 15% of the Company’s common stock without the approval of the Board of Directors under certain circumstances. The Company has designated 250,000 shares of Series C Junior Participating preferred stock for issuance in connection with the stockholder rights plan. As of December 31, 2017, no Series C Junior Participating preferred stock has been issued. The expiration date of the rights issued under the stockholder rights plan is October 27, 2019.

Note 11. Stockholders’ Equity

Sales Agreement

On May 5, 2016, the Company entered into Amendment No. 2 to the Controlled Equity Offering^SM Sales Agreement, dated August 31, 2012, as previously amended on March 21, 2014, (together, the “Prior Cantor Agreement”) with Cantor Fitzgerald & Co. (“Cantor”) that provides for the issuance and sale of shares of the Company’s common stock having an aggregate offering price of up to $132.2 million through Cantor over the term of the Amended Cantor Agreement. As a result of Amendment No. 2, at May 5, 2016, the Company had $70 million of common stock available to be sold under the Amended Cantor Agreement. During the years ended December 31, 2017 and 2016, 11.0 million and 3.5 million shares, respectively, of the Company’s common stock were sold under the Prior Cantor Agreement for net proceeds of $30.3 million and $22.0 million, respectively.

On August 4, 2017, the Company entered into Amendment No. 3 to the Prior Cantor Agreement (together, the “Amended Cantor Agreement”). The Amended Cantor Agreement became effective on January 8, 2018, and provides for the issuance and sale of shares of the Company’s common stock having an aggregate offering price of up to $70.0 million through Cantor, which amount includes the $31.4 million of unsold shares of common stock available for sale under the Prior Cantor Agreement immediately prior to the effectiveness of the Amended Cantor Agreement. Under the Amended Cantor Agreement, Cantor also acts as the Company’s sales agent and receives compensation based on an aggregate of 2% of the gross proceeds on the sale price per share of its common stock. The issuance and sale of these shares by the Company pursuant to the Amended Cantor Agreement are deemed an “at-the-market” offering and are registered under the Securities Act of 1933, as amended.

Stockholder Rights Plan

In October 2009, the Company’s Board of Directors adopted an amendment to its 1999 stockholder rights plan, commonly referred to as a “poison pill,” to reduce the exercise price, extend the expiration date and revise certain definitions under the plan. The stockholder rights plan is intended to deter hostile or coercive attempts to acquire the Company. The stockholder rights plan enables stockholders to acquire shares of the Company’s common stock, or the common stock of an acquirer, at a substantial discount to the public market price should any person or group acquire more than 15% of the Company’s common stock without the approval of the Board of Directors under certain circumstances. The Company has designated 250,000 shares of Series C Junior Participating preferred stock for issuance in connection with the stockholder rights plan. As of December 31, 2017, no Series C Junior Participating preferred stock has been issued. The expiration date of the rights issued under the stockholder rights plan is October 27, 2019.