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Share-Based Payments
 9 Months Ended
Sep. 30, 2019
Share-based Payment Arrangement [Abstract]  
Share-Based Payments Share-Based Payments
2013 Stock Incentive Plan
In June 2013, our Board of Directors adopted and, in July 2013 our stockholders approved, the 2013 Stock Incentive Plan, or the 2013 Plan. The 2013 Plan became effective upon the closing of our initial public offering and provides for the grant of stock options and other stock-based awards. Following the adoption of the 2013 Plan, we granted no further stock options or other stock-based awards under the 2007 Stock Incentive Plan, or the 2007 Plan. Any stock options or stock-based awards outstanding under the 2007 Plan at the time of adoption of the 2013 Plan remained outstanding and effective. As of September 30, 2019, the total number of shares reserved under the 2007 Plan and the 2013 Plan was 9,392,668, and we had 2,248,177 shares available for future issuance under the 2013 Plan.
Stock options
The following table presents stock option activity for the nine months ended September 30, 2019:
Number of
Stock Options
Weighted-Average
Exercise Price
Outstanding at December 31, 20185,416,069  $60.10  
Granted1,577,686  55.85  
Exercised(253,323) 34.51  
Forfeited/Expired(554,497) 67.59  
Outstanding at September 30, 20196,185,935  $59.39  
Exercisable at September 30, 20193,441,309  $58.90  
Vested and expected to vest at September 30, 20196,185,935  $59.39  
At September 30, 2019, there was approximately $102.3 million of total unrecognized compensation expense related to unvested stock option awards, which we expect to recognize over a weighted-average period of approximately 2.7 years.
Restricted stock units
The following table presents restricted stock unit, or RSU, activity for the nine months ended September 30, 2019:
Number of
Stock Units
Weighted-Average
Grant Date Fair 
Value
Unvested shares at December 31, 2018532,144  $75.45  
Granted437,227  57.25  
Vested(160,703) 69.52  
Forfeited(75,568) 71.79  
Unvested shares at September 30, 2019733,100  $66.28  
As of September 30, 2019, there was approximately $32.1 million of total unrecognized compensation expense related to RSUs, which we expect to recognize over a weighted-average period of approximately 1.8 years.
Performance-based stock units
The following table presents performance-based stock unit, or PSU, activity for the nine months ended September 30, 2019:
Number of
Stock Units
Weighted-Average
Grant Date Fair 
Value
Unvested shares at December 31, 2018169,031  $52.67  
Granted180,761  58.64  
Vested(167,031) 52.36  
Unvested shares at September 30, 2019182,761  $58.87  
Stock-based compensation expense associated with these PSUs is recognized if the underlying performance condition is considered probable of achievement using our management’s best estimates.
As of September 30, 2019, there was approximately $2.0 million of total unrecognized compensation expense related to PSUs with performance-based vesting criteria that are considered probable of achievement, which we expect to recognize over a
weighted-average period of 0.6 years, and $7.2 million of total unrecognized compensation expense related to PSUs with performance-based vesting criteria that are considered not probable of achievement.
Market-based stock units
The following table presents market-based stock unit, or MSU, activity for the nine months ended September 30, 2019:
Number of
Stock Units		Weighted-Average
Grant Date Fair
Value
Unvested shares at December 31, 2018—  $—  
Granted42,695  41.50  
Unvested shares at September 30, 201942,695  $41.50  
The fair value of MSUs are estimated using a Monte Carlo simulation model. Assumptions and estimates utilized in the model include the risk-free interest rate, dividend yield, expected stock volatility and the estimated period to achievement of the market condition. As of September 30, 2019, there was approximately $1.1 million of total unrecognized compensation expense related to MSUs, which we expect to recognize over the remaining derived service period of 1.0 year.
2013 Employee Stock Purchase Plan
In June 2013, our Board of Directors adopted, and in July 2013 our stockholders approved, the 2013 Employee Stock Purchase Plan, or the 2013 ESPP. We issued 77,981 and 53,255 shares of common stock during the nine months ended September 30, 2019 and 2018, respectively, under the 2013 ESPP. The 2013 ESPP provides participating employees with the opportunity to purchase up to an aggregate of 327,272 shares of our common stock. As of September 30, 2019, we had 82,555 shares of common stock available for future issuance under the 2013 ESPP.
Stock-based compensation expense
Stock-based compensation expense by award type included within the condensed consolidated statements of operations is as follows (in thousands):
Three Months Ended
September 30,		Nine Months Ended
September 30,
2019201820192018
Stock options$11,552  $12,953  $37,065  $38,736  
Restricted stock units4,917  3,338  14,708  7,940  
Employee stock purchase plan350  246  1,070  838  
Other stock awards1,769  7,656  2,400  7,656  
Total stock-based compensation expense$18,588  $24,193  $55,243  $55,170  
Expenses related to stock options and stock-based awards were allocated as follows in the condensed consolidated statements of operations (in thousands):
Three Months Ended
September 30,		Nine Months Ended
September 30,
2019201820192018
Research and development expense$9,860  $13,399  $29,969  $31,706  
Selling, general and administrative expense8,728  10,794  25,274  23,464  
Total stock-based compensation expense$18,588  $24,193  $55,243  $55,170