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Stock-Based Compensation | 12 Months Ended
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Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Stock-Based Compensation | Note 11 — Stock-Based Compensation We issue stock-based awards from our equity incentive plans, which are more fully described in Note 9. Stock-based compensation expense was recognized as follows (in thousands):
|  | Year Ended December 31, |
|  | 2017 |  |  | 2016 |  |  | 2015 |
Cost of goods sold |  | $ | 2,333 |  |  | $ | 1,620 |  |  | $ | 1,142 |
Research and development |  |  | 21,252 |  |  |  | 13,093 |  |  |  | 9,207 |
General and administrative |  |  | 13,030 |  |  |  | 11,137 |  |  |  | 9,320 |
Total stock-based compensation |  | $ | 36,615 |  |  | $ | 25,850 |  |  | $ | 19,669 |
As of December 31, 2017, total unrecognized compensation costs of $211.8 million related to unvested stock-based compensation arrangements are expected to be recognized as expense over a weighted-average period of 2.1 years. Black-Scholes Assumptions The following table lists the Black-Scholes option-pricing model assumptions used to calculate the fair value of employee and director stock options:
|  | Year Ended December 31, |
|  | 2017 |  |  | 2016 |  |  | 2015 |
Average risk-free interest rate |  |  | 2.0 | % |  |  | 1.4 | % |  |  | 1.6 | %
Dividend yield |  |  | 0.0 | % |  |  | 0.0 | % |  |  | 0.0 | %
Average volatility factor |  |  | 54.2 | % |  |  | 51.7 | % |  |  | 50.8 | %
Average weighted average expected life |  | 5.3 years |  |  | 5.3 years |  |  | 5.3 years |
The average risk-free interest rate is based on the U.S. treasury yield curve in effect at the time of grant for periods commensurate with the expected life of the stock-based award. We have never paid dividends, nor do we expect to pay dividends in the foreseeable future; therefore, we used a dividend yield of 0.0%. Our estimate of expected volatility is based on the daily historical trading data of our common stock at the time of grant over a historical period commensurate with the expected life of the stock-based award. We estimated the weighted-average expected life based on the contractual and vesting terms of the stock options, as well as historical cancellation and exercise data. Stock-based compensation expense resulting from our ESPP was not significant in the years ended December 31, 2017, 2016, and 2015. Summary of Stock Option Activity The table below presents a summary of stock option activity under our equity incentive plans (in thousands, except for price per share and contractual life information):
|  | Number of Shares |  |  | Weighted- Average Exercise Price per Share |  |  | Weighted- Average Remaining Contractual Life (in Years) |  |  | Aggregate Intrinsic Value(1) |
Outstanding at December 31, 2016 |  |  | 19,413 |  |  | $ | 12.01 |  |  |  |  |  |  |  |  |
Options granted |  |  | 5,322 |  |  |  | 40.56 |  |  |  |  |  |  |  |  |
Options exercised |  |  | (5,284 | ) |  |  | 10.88 |  |  |  |  |  |  |  |  |
Options forfeited & canceled |  |  | (625 | ) |  |  | 13.69 |  |  |  |  |  |  |  |  |
Outstanding at December 31, 2017 |  |  | 18,826 |  |  | $ | 20.35 |  |  |  | 5.15 |  |  | $ | 741,257 |
Exercisable at December 31, 2017 |  |  | 10,504 |  |  | $ | 11.95 |  |  |  | 3.57 |  |  | $ | 501,818 |
(1) | Aggregate intrinsic value represents the difference between the exercise price of the option and the closing market price of our common stock on December 31, 2017.
The weighted-average grant-date fair value per share of options granted during the years ended December 31, 2017, 2016, and 2015 was $20.08, $6.54, and $6.55, respectively. The total intrinsic value of options exercised during the years ended December 31, 2017, 2016, and 2015 was $84.0 million, $17.9 million, and $20.5 million, respectively. The estimated fair value of options vested during the years ended December 31, 2017, 2016, and 2015 was $19.3 million, $16.7 million, and $16.8 million, respectively. Summary of RSU Activity A summary of RSU award activity is as follows (in thousands except for per share amounts):
|  | Units Issued |  |  | Weighted- Average Grant Date Fair Value |  |  | Aggregate Intrinsic Value(1) |
Balance at December 31, 2016 |  |  | 2,248 |  |  | $ | 13.53 |  |  |  |  |
Granted |  |  | 1,405 |  |  |  | 54.14 |  |  |  |  |
Vested and released |  |  | (875 | ) |  |  | 14.05 |  |  |  |  |
Forfeited and canceled |  |  | (200 | ) |  |  | 13.14 |  |  |  |  |
Balance at December 31, 2017 |  |  | 2,578 |  |  | $ | 35.52 |  |  | $ | 153,941 |
(1) | Aggregate intrinsic value represents the difference between the grant price of the award, which is zero, and the closing market price of our common stock on December 31, 2017.

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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