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Employee Benefit Plans and Stockholders' Equity (Tables) | 12 Months Ended
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Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Summary of Stock Option Activity | A summary of our stock option activity and related information for the twelve months ended December 31, 2018 is as follows:
| Number of Shares (in millions) |  | Weighted- Average Exercise Price per Share |  | Weighted- Average Remaining Contractual Term (in years) |  | Aggregate Intrinsic Value (in millions)
Outstanding at December 31, 2017 | 0.4 |  |  | $ | 6.71 |  |  |  |  |
Exercised | (0.3 | ) |  | 6.34 |  |  |  |  |
Forfeited | — |  |  | — |  |  |  |  |
Outstanding at December 31, 2018 | 0.1 |  |  | 8.02 |  |  | 0.95 |  | $ | 9.6 |
Exercisable at December 31, 2018 | 0.1 |  |  | $ | 8.02 |  |  | 0.95 |  | $ | 9.6 |
Schedule of Intrinsic Value of Options Exercised and Fair Value of Options Vested | The total intrinsic value of stock options exercised as of the date of exercise was as follows:
| Years Ended December 31,
(In millions) | 2018 |  | 2017 |  | 2016
Intrinsic value of options exercised | $ | 30.0 |  |  | $ | 21.6 |  |  | $ | 39.9 |
Schedule of Share-Based Compensation Expenses | The following table summarizes share-based compensation expense related to restricted stock units, stock options, and employee stock purchases under the ESPP for the twelve months ended December 31, 2018, 2017 and 2016:
| Years Ended December 31,
(In millions) | 2018 |  | 2017 |  | 2016
Cost of sales | $ | 9.2 |  |  | $ | 9.6 |  |  | $ | 12.0 |
Research and development | 33.0 |  |  | 37.5 |  |  | 39.8 |
Selling, general and administrative | 59.7 |  |  | 59.1 |  |  | 59.0 |
Total share-based compensation expense included in net loss | $ | 101.9 |  |  | $ | 106.2 |  |  | $ | 110.8 |
Schedule of Valuation Assumptions for Employee Stock Purchase Plan | We estimate the fair value of stock options granted and ESPP purchase rights on the date of grant using the Black-Scholes option pricing model and the assumptions below. We did not have any stock option grants during the twelve months ended December 31, 2018, 2017 and 2016.
| Years Ended December 31,
ESPP | 2018 |  | 2017 |  | 2016
Risk free interest rate | 1.55 – 2.25 |  |  | 0.75 – 1.12 |  |  | 0.46 – 0.57 |
Dividend yield | — | % |  | — | % |  | — | %
Expected volatility of DexCom common stock | 0.50 – 0.67 |  |  | 0.33 – 0.56 |  |  | 0.33 – 0.57 |
Expected life (in years) | 1 |  |  | 1 |  |  | 1 |
Schedule of Restricted Stock Units Activity | A summary of our RSU activity for the twelve months ended December 31, 2018, 2017 and 2016 is as follows:
(In millions except weighted average grant date fair value) | Shares |  | Weighted Average Grant Date Fair Value |  | Aggregate Intrinsic Value
Nonvested at December 31, 2015 | 4.1 |  |  | $ | 50.60 |  |  |
Granted | 1.9 |  |  | 68.16 |  |  |
Vested | (2.1 | ) |  | 44.95 |  |  |
Forfeited | (0.2 | ) |  | 56.37 |  |  |
Nonvested at December 31, 2016 | 3.7 |  |  | 62.51 |  |  | $ | 218.6 |
Granted | 1.3 |  |  | 75.78 |  |  |
Vested | (1.9 | ) |  | 58.92 |  |  |
Forfeited | (0.4 | ) |  | 67.97 |  |  |
Nonvested at December 31, 2017 | 2.7 |  |  | 70.68 |  |  | 154.5 |
Granted | 1.7 |  |  | 66.07 |  |  |
Vested | (1.4 | ) |  | 68.44 |  |  |
Forfeited | (0.3 | ) |  | 68.56 |  |  |
Nonvested at December 31, 2018 | 2.7 |  |  | $ | 69.19 |  |  | $ | 319.0 |
Schedule of Stock Options Reserved for Future Issuance | Reserved Shares Shares of common stock reserved for future issuance were as follows as of the dated indicated:
| December 31,
(In millions) | 2018 |  | 2017
Stock options and awards under our plans: |  |  |
Stock options granted and outstanding | 0.1 |  |  | 0.4 |
Unvested restricted stock units | 2.7 |  |  | 2.7 |
Reserved for future grant | 3.2 |  |  | 4.7 |
Employee Stock Purchase Plan | 1.1 |  |  | 1.3 |
Total | 7.1 |  |  | 9.1 |

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