Stockholders' Equity	12 Months Ended
Dec. 31, 2018
Equity [Abstract]
Stockholders' Equity	Stockholders' Equity Common Stock and Warrants As of December 31, 2018, the Company was authorized to issue 500 million shares of common stock. Dividends on common stock will be paid when, and if, declared by the board of directors. Each holder of common stock is entitled to vote on all matters that are appropriate for stockholder voting and is entitled to one vote for each share held. On February 15, 2018, the Company announced the pricing of an underwritten offering of 19,354,839 shares of its common stock at $15.50 per share, resulting in gross proceeds of $300.0 million. Under the terms of the underwriting agreement, the Company granted the underwriters an option, exercisable for 30 days after February 16, 2018, to purchase up to an additional 2,903,225 shares of the Company’s common stock, which was exercised with respect to 885,000 shares of the Company’s common stock at a purchase price of $15.50 per share. The Company received net proceeds of $294.6 million from these offerings, after deducting underwriting discounts and commissions and offering expenses payable by the Company. In April 2018, 453,214 warrants were exercised at $7.98 per share of common stock resulting in gross cash proceeds of $3.6 million. On June 7, 2018, the Company’s stockholders approved an amendment to the Company’s Restated Certificate of Incorporation to increase the number of shares of common stock, par value $0.01 per share, that the Company is authorized to issue from 250,000,000 shares to 500,000,000 shares. In July 2017, the Company entered into an underwriting agreement whereby the Company issued and sold 21,122,449 shares at a price to the public of $12.25 per share, resulting in gross proceeds of $258.8 million, before deducting underwriting discounts and commissions and offering expenses payable by the Company. The offering closed on July 18, 2017 and the Company received net proceeds, after deducting underwriting discounts and commissions and offering expenses payable by the Company of $243.0 million. As discussed in ‘‘— Note 12. Debt’’, in December 2016, the Company issued $250 million aggregate principal amount of 3.0% unsecured Convertible Senior Notes due 2023 (the ‘‘Convertible Notes), in a private offering. The Notes will mature on December 15, 2023, unless earlier repurchased, redeemed, or converted in accordance with their terms. The Notes are convertible at the option of the holders, under certain circumstances and during certain periods, into cash, shares of the Company’s common stock, par value $0.01 per share (‘‘common stock’’), or a combination thereof. Prior to the close of business on the business day immediately preceding September 15, 2023, the Notes are convertible at the option of the holders of the Notes only under certain conditions. On or after September 15, 2023, until the close of business on the second business day immediately preceding the maturity date, holders of the Notes may convert their Notes at their option at the conversion rate then in effect, irrespective of these conditions. The Company will settle conversions of the Notes by paying or delivering, as the case may be, cash, shares of common stock, or a combination of cash and shares of common stock, at the Company’s election. The conversion rate will initially be 163.3987 shares of common stock per $1,000 principal amount of Notes (equivalent to an initial conversion price of approximately $6.12 per share of common stock). The conversion rate is subject to customary adjustments upon the occurrence of certain events. During the first quarter of 2019, the Company entered into separate, privately negotiated exchange agreements with a limited number of holders (the “Holders”) of the Convertible Notes. Under the terms of the exchange agreements (the “Exchange Agreements”), the Holders agreed to exchange an aggregate principal amount of approximately $184.6 million of Convertible Notes held by them in exchange for an aggregate of approximately 33.0 million shares of the our common stock, par value $0.01 per share. In addition, pursuant to the Exchange Agreements, the Company made aggregate cash payments of approximately $0.7 million to the Holders to satisfy accrued and unpaid interest to the closing date of the transaction, along with cash in lieu of fractional shares. Nonqualified Cash Plan The Company's Deferral Plan, (the "Deferral Plan") provides certain key employees and members of the Board of Directors as selected by the Compensation Committee, with an opportunity to defer the receipt of such participant's base salary, bonus and director's fees, as applicable. The Deferral Plan is intended to be a nonqualified deferred compensation plan that complies with the provisions of Section 409A of the Internal Revenue Code of 1986 as amended. The Company had a deferred compensation investment balance of $2.7 million and $2.2 million as of December 31, 2018 and 2017, respectively, with corresponding approximate amounts of liability. Deferral Plan investment assets are classified as trading securities and are recorded at fair value with changes in the investments' fair value recognized in AOCI in the period they occur. Deferred compensation liability amounts under the Deferral Plan are included in other long-term liabilities. Equity Incentive Plans The Company’s Equity Incentive Plans consist of the Amended and Restated 2007 Equity Incentive Plan (the ‘‘Plan’’) and the 2007 Director Option Plan (the ‘‘2007 Director Plan’’). The Plan provides for the granting of restricted stock and options to purchase common stock in the Company to employees, advisors and consultants at a price to be determined by the Company’s board of directors. The Plan is intended to encourage ownership of stock by employees and consultants of the Company and to provide additional incentives for them to promote the success of the Company’s business. The 2007 Director Plan is intended to promote the recruiting and retention of highly qualified eligible directors and strengthen the commonality of interest between directors and stockholders by encouraging ownership of common stock of the Company. Under the provisions of each plan, no option will have a term in excess of 10 years. The Board of Directors, or its committee, is responsible for determining the individuals to be granted options, the number of options each individual will receive, the option price per share, and the exercise period of each option. Options granted pursuant to the Plan generally vest 25% on the first year anniversary date of grant plus an additional 1/48th for each month thereafter and may be exercised in whole or in part for 100% of the shares vested at any time after the date of grant. Options under the 2007 Director Plan may be granted to new directors upon joining the Board and vest in the same manner as options under the Plan. In addition, options are granted to independent directors at each annual meeting of stockholders and vest on the date of the annual meeting of stockholders of the Company in the year following the year during which the options were granted. As of December 31, 2018, the Company has reserved up to 9,457,741 shares for issuance under the Plan and the 2007 Director Plan.

Stockholders' Equity

Common Stock and Warrants

As of December 31, 2018, the Company was authorized to issue 500 million shares of common stock. Dividends on common stock will be paid when, and if, declared by the board of directors. Each holder of common stock is entitled to vote on all matters that are appropriate for stockholder voting and is entitled to one vote for each share held.

On February 15, 2018, the Company announced the pricing of an underwritten offering of 19,354,839 shares of its common stock at $15.50 per share, resulting in gross proceeds of $300.0 million. Under the terms of the underwriting agreement, the Company granted the underwriters an option, exercisable for 30 days after February 16, 2018, to purchase up to an additional 2,903,225 shares of the Company’s common stock, which was exercised with respect to 885,000 shares of the Company’s common stock at a purchase price of $15.50 per share. The Company received net proceeds of $294.6 million from these offerings, after deducting underwriting discounts and commissions and offering expenses payable by the Company.

In April 2018, 453,214 warrants were exercised at $7.98 per share of common stock resulting in gross cash proceeds of $3.6 million.

On June 7, 2018, the Company’s stockholders approved an amendment to the Company’s Restated Certificate of Incorporation to increase the number of shares of common stock, par value $0.01 per share, that the Company is authorized to issue from 250,000,000 shares to 500,000,000 shares.

In July 2017, the Company entered into an underwriting agreement whereby the Company issued and sold 21,122,449 shares at a price to the public of $12.25 per share, resulting in gross proceeds of $258.8 million, before deducting underwriting discounts and commissions and offering expenses payable by the Company. The offering closed on July 18, 2017 and the Company received net proceeds, after deducting underwriting discounts and commissions and offering expenses payable by the Company of $243.0 million.

As discussed in ‘‘— Note 12. Debt’’, in December 2016, the Company issued $250 million aggregate principal amount of 3.0% unsecured Convertible Senior Notes due 2023 (the ‘‘Convertible Notes), in a private offering. The Notes will mature on December 15, 2023, unless earlier repurchased, redeemed, or converted in accordance with their terms. The Notes are convertible at the option of the holders, under certain circumstances and during certain periods, into cash, shares of the Company’s common stock, par value $0.01 per share (‘‘common stock’’), or a combination thereof. Prior to the close of business on the business day immediately preceding September 15, 2023, the Notes are convertible at the option of the holders of the Notes only under certain conditions. On or after September 15, 2023, until the close of business on the second business day immediately preceding the maturity date, holders of the Notes may convert their Notes at their option at the conversion rate then in effect, irrespective of these conditions. The Company will settle conversions of the Notes by paying or delivering, as the case may be, cash, shares of common stock, or a combination of cash and shares of common stock, at the Company’s election. The conversion rate will initially be 163.3987 shares of common stock per $1,000 principal amount of Notes (equivalent to an initial conversion price of approximately $6.12 per share of common stock). The conversion rate is subject to customary adjustments upon the occurrence of certain events.

During the first quarter of 2019, the Company entered into separate, privately negotiated exchange agreements with a limited number of holders (the “Holders”) of the Convertible Notes. Under the terms of the exchange agreements (the “Exchange Agreements”), the Holders agreed to exchange an aggregate principal amount of approximately $184.6 million of Convertible Notes held by them in exchange for an aggregate of approximately 33.0 million shares of the our common stock, par value $0.01 per share. In addition, pursuant to the Exchange Agreements, the Company made aggregate cash payments of approximately $0.7 million to the Holders to satisfy accrued and unpaid interest to the closing date of the transaction, along with cash in lieu of fractional shares.

Nonqualified Cash Plan

The Company's Deferral Plan, (the "Deferral Plan") provides certain key employees and members of the Board of Directors as selected by the Compensation Committee, with an opportunity to defer the receipt of such participant's base salary, bonus and director's fees, as applicable. The Deferral Plan is intended to be a nonqualified deferred compensation plan that complies with the provisions of Section 409A of the Internal Revenue Code of 1986 as amended.

The Company had a deferred compensation investment balance of $2.7 million and $2.2 million as of December 31, 2018 and 2017, respectively, with corresponding approximate amounts of liability.

Deferral Plan investment assets are classified as trading securities and are recorded at fair value with changes in the investments' fair value recognized in AOCI in the period they occur. Deferred compensation liability amounts under the Deferral Plan are included in other long-term liabilities.

Equity Incentive Plans

The Company’s Equity Incentive Plans consist of the Amended and Restated 2007 Equity Incentive Plan (the ‘‘Plan’’) and the 2007 Director Option Plan (the ‘‘2007 Director Plan’’). The Plan provides for the granting of restricted stock and options to purchase common stock in the Company to employees, advisors and consultants at a price to be determined by the Company’s board of directors. The Plan is intended to encourage ownership of stock by employees and consultants of the Company and to provide additional incentives for them to promote the success of the Company’s business. The 2007 Director Plan is intended to promote the recruiting and retention of highly qualified eligible directors and strengthen the commonality of interest between directors and stockholders by encouraging ownership of common stock of the Company. Under the provisions of each plan, no option will have a term in excess of 10 years. The Board of Directors, or its committee, is responsible for determining the individuals to be granted options, the number of options each individual will receive, the option price per share, and the exercise period of each option. Options granted pursuant to the Plan generally vest 25% on the first year anniversary date of grant plus an additional 1/48th for each month thereafter and may be exercised in whole or in part for 100% of the shares vested at any time after the date of grant. Options under the 2007 Director Plan may be granted to new directors upon joining the Board and vest in the same manner as options under the Plan. In addition, options are granted to independent directors at each annual meeting of stockholders and vest on the date of the annual meeting of stockholders of the Company in the year following the year during which the options were granted. As of December 31, 2018, the Company has reserved up to 9,457,741 shares for issuance under the Plan and the 2007 Director Plan.