Stockholders' Equity | Stockholders ’ Equity Common Stock As of June 30, 2019 , there were 100,000,000 shares of common stock authorized, and 65,398,801 shares issued and outstanding. As of December 31, 2018 , there were 100,000,000 shares of common stock authorized, and 63,676,229 shares issued and outstanding. The par value for common shares is $0.001 per share. On June 6, 2019, our stockholders approved an amendment to our charter to increase the number of shares of our common stock that we are authorized to issue from 100,000,000 to 200,000,000 shares. We expect to file a charter amendment with the State of Delaware to effectuate this increase in the number of shares of our common stock that we are authorized to issue. Preferred Stock As of June 30, 2019 and December 31, 2018 , there were 5,000,000 shares of preferred stock authorized, and zero shares issued and outstanding. The par value for preferred shares is $0.001 per share. Stock Repurchase Program From 2012 through 2018, the Company had a stock repurchase program in place pursuant to which the Company was authorized to repurchase shares of its common stock, in the open market or privately negotiated transactions, at times and prices considered appropriate by the Board of Directors depending upon prevailing market conditions and other corporate considerations. The timing and actual number of shares repurchased depend on a variety of factors including the timing of open trading windows, price, corporate and regulatory requirements and other market conditions. The program was discontinued at the end of 2018. The Company may or may not enter into a new stock repurchase program in the future. Stock-Based Compensation The Company follows FASB ASC 718-10, “Stock Compensation,” which addresses the accounting for transactions in which an entity exchanges its equity instruments for goods or services, with a primary focus on transactions in which an entity obtains employee services in share-based payment transactions. ASC 718-10 requires measurement of the cost of employee services received in exchange for an award of equity instruments based on the grant-date fair value of the award. Incremental compensation costs arising from subsequent modifications of awards after the grant date must be recognized. The per share weighted average fair value of stock options granted was $11.74 and $8.15 during the three months ended June 30, 2019 and 2018, respectively and $11.28 and $6.26 during the six months ended June 30, 2019 and 2018, respectively. The fair value of each option grant is estimated on the date of grant using the Black-Scholes option-pricing model with the following weighted average assumptions: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Dividend yield 0.0% 0.0% 0.0% 0.0% Risk-free interest rate 1.88% - 2.36% 2.6% - 2.8% 1.88% - 2.57% 2.5% - 2.8% Expected life (in years) 5 5 5 5 Historical volatility 43.42% - 43.69% 43.7% - 48.4% 43.42% - 43.85% 43.7% - 48.4% A description of the methods used in the significant assumptions used to estimate the fair value of stock-based compensation awards follows: Dividend yield – The Company uses 0% as it has never issued dividends and does not anticipate issuing dividends in the near term. Risk-free interest rate – The Company uses the market yield on U.S. Treasury securities at five years with constant maturity, representing the current expected life of stock options in years. Expected life – The Company uses historical data to estimate the expected life of a stock option. Historical volatility – The Company uses a trailing five year from grant date to determine volatility. Stock Option Plans During 1998, the Company established the Stock Option and Restricted Stock Purchase Plan (the “1998 Plan”). Under the 1998 Plan, the Board of Directors could issue incentive stock options or nonqualified stock options or other equity-based awards in respect of up to 5,850,000 shares of common stock. The 2000 Stock Incentive Plan (the “2000 Plan”) succeeded the 1998 Plan. Under the 2000 Plan, the options which had been outstanding under the 1998 Plan were incorporated in the 2000 Plan increasing the number of shares available for issuance under the 2000 Plan by approximately 4,150,000 , thereby reserving for issuance 10,000,000 shares of common stock in the aggregate. The Company established the 2009 Stock Incentive Plan (the “2009 Plan”) as a successor to the 2000 Plan. Under the 2009 Plan, the options which had been outstanding under the 2000 Plan were incorporated into the 2009 Plan and the Company increased the number of shares available for issuance under the plan by 6,000,000 . The Company amended the 2009 Plan (the “Amended 2009 Plan”) effective June 7, 2012. The Amended 2009 Plan increased the number of shares authorized for issuance under the plan by an additional 4,250,000 . On June 2, 2017, the Company's Board of Directors amended and restated the Amended 2009 Plan effective April 30, 2017. The amended and restated plan increased the number of shares authorized for issuance under the plan by an additional 4,000,000 . On April 11, 2019, the Company's Board of Directors adopted, and on June 6, 2019, the Company's stockholders approved, the 2019 Stock Incentive Plan ("2019 Stock Incentive Plan") to replace the Amended and Restated 2009 Plan, which was set to expire under its terms on June 9, 2019. Under the 2019 Stock Incentive Plan, the number of shares underlying options and other equity awards which remain outstanding, as well as the number of shares that remained available for grant under the 2009 Plan and under the 2000 Plan were incorporated, as of June 6, 2019 into the 2019 Stock Incentive Plan. In addition, under the 2019 Stock Incentive Plan, 4,250,000 new shares were authorized for issuance. The number of shares authorized for issuance under the 2019 Stock Incentive Plan, the Amended 2009 Plan, and the 2000 Plan is 32,067,744 shares in the aggregate. Options to acquire common stock granted thereunder have 10 -year terms. As of June 30, 2019 , approximately 4.9 million shares of common stock remained available for issuance under the 2019 Stock Incentive Plan (taking into account all option exercises and other equity award settlements through June 30, 2019 ). Employee Stock Purchase Plan In June 2010, the Company’s stockholders approved the 2010 Employee Stock Purchase Plan with 1,000,000 shares of common stock initially reserved for issuance. Subject to stockholder approval, which was obtained on June 2, 2017, the Company's Board of Directors amended and restated the 2010 Employee Stock Purchase Plan effective April 30, 2017. The amended and restated plan increased the number of shares authorized for issuance under the plan by an additional 1,000,000 , thereby reserving for issuance 2,000,000 shares of common stock in the aggregate. On April 11, 2019, the Company's Board of Directors adopted, and on June 6, 2019, the Company's stockholders approved, the 2019 Employee Stock Purchase Plan (the "2019 Employee Stock Purchase Plan") to replace the Amended and Restated 2010 Employee Stock Purchase Plan which was set to expire under its terms in June 2020. On April 22, 2019, at the Annual Meeting of the Stockholders, the Company's stockholders voted to approve the adoption of the 2019 Employee Stock Purchase Plan. There were 1,000,000 shares authorized to be reserved for issuance under the 2019 Employee Stock Purchase Plan. As of June 30, 2019 , approximately 1.0 million shares of common stock remained available for issuance under the 2019 Employee Stock Purchase Plan (taking into account all share purchases through June 30, 2019 ). Inducement Plan During January 2018, the Company established the Inducement Plan (the “2018 Plan”). Under the 2018 Plan, the Board of Directors can issue nonqualified stock options or other equity-based awards in respect of up to 1,500,000 shares of common stock. On April 25, 2018, the Company's Board of Directors amended and restated the 2018 Plan (the "Amended 2018 Plan"). The Amended 2018 Plan increased the number of shares authorized for issuance under the plan by an additional 500,000 shares, and subsequently the Board of Directors has approved and ratified, effective as of July 31, 2018, October 29, 2018 and February 13, 2019, increases of the number of shares authorized for issuance under the Amended 2018 Plan by 500,000 , 250,000 and 618,048 shares, respectively, constituting 3,368,048 shares of common stock in the aggregate being reserved for issuance pursuant to grants under the Amended 2018 Plan. As of June 30, 2019 , approximately 0.3 million shares of common stock remained available for issuance under the Amended 2018 Plan (taking into account all option exercises and other equity award settlements through June 30, 2019 ). Stock Option Activity A summary of the Company’s stock option activity and weighted average exercise prices follows: Stock Option Activity Weighted Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Options (in thousands) Weighted Average Exercise Price Balance outstanding at December 31, 2018 6,266 $ 12.13 Granted 1,035 27.59 Exercised (890 ) 10.23 Cancelled or expired (178 ) 14.38 Balance outstanding at June 30, 2019 6,233 $ 14.91 6.92 $ 81,143 Options vested and expected to vest 5,435 $ 14.00 6.61 $ 75,377 Options exercisable at June 30, 2019 3,168 $ 12.07 5.09 $ 49,793 The total fair value of stock options exercised during the six months ended June 30, 2019 was approximately $4.2 million . As of June 30, 2019 , there was approximately $21.9 million of total unrecognized compensation cost related to nonvested share-based compensation arrangements. That cost is expected to be recognized over a weighted average period of approximately 2.7 years. The following table summarizes information about outstanding and vested stock options as of June 30, 2019 : Options Outstanding Options Exercisable Range of Exercise Prices Number of Shares Outstanding (in thousands) Weighted-Average Remaining Contractual Life (Years) Weighted-Average Exercise Price Number of Shares (in thousands) Weighted-Average Exercise Price $5.15 - $7.60 781 6.81 $ 7.39 399 $ 7.32 $7.95 - $10.05 768 5.46 9.46 663 9.50 $10.13 - $11.95 768 6.17 10.84 584 10.53 $11.96 - $12.32 76 2.03 12.09 76 12.09 $12.45 - $12.45 843 8.63 12.45 203 12.45 $12.46 - $14.3 876 4.40 13.60 655 13.37 $14.5 - $17.88 664 6.59 16.17 345 16.60 $18.09 - $25.95 675 8.34 22.06 243 20.35 $28.18 - $28.79 453 9.68 28.28 — — $29.55 - $29.55 329 9.78 29.55 — — 6,233 6.92 $ 14.91 3,168 $ 12.07 Restricted Stock Unit Activity A summary of the Company’s restricted stock units (“RSUs”) activity and weighted average exercise prices follows: Restricted Stock Unit Activity Number of Shares (in thousands) Weighted Average Grant Date Fair Value (Per Share) Aggregate Fair Value (in thousands) Balance outstanding at December 31, 2018 2,690 $ 15.81 $ — Awarded 1,331 27.45 — Released (754 ) 13.47 — Forfeited (214 ) 17.53 — Non-vested and outstanding at June 30, 2019 3,053 $ 20.79 $ 84,840 Expected to vest 2,043 $ 19.38 $ 56,769 RSUs granted to employees generally vest over a three to four -year period or upon achievement of certain performance conditions. In accordance with ASU 2017-09, as of June 30, 2019 , total unrecognized compensation cost, adjusted for estimated forfeitures, related to nonvested RSUs was approximately $59.3 million and the weighted-average remaining vesting period was 3.0 years. As of June 30, 2019 , the Company accrued approximately $2.9 million and $4.8 million in cash awards to be settled in shares of the Company's stock and recorded a corresponding expense, which is included as a component of stock-based compensation expense in the accompanying condensed consolidated financial statements for the three and six months ended June 30, 2019 , respectively. Stock-based compensation expense recognized in the Company’s condensed consolidated statements of operations and cash flows was $9.3 million and $3.8 million for the three months ended June 30, 2019 and 2018, respectively, and $16.5 million and $6.3 million for the six months ended June 30, 2019 |