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Equity Incentive Plans and Stock-Based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Schedule of Share Based Compensation Stock Options Awards, Number of Shares Activity [Table Text Block] | A summary of shares available for grant under the Company’s plans is as follows:
| Shares Available for Grant
Shares available as of December 31, 2017 | 5,051,147 |
Increase in shares approved for issuance | 5,500,000 |
Stock options granted | (621,479 | )
Stock options forfeited | 765,382 |
Nonvested equity stock and stock units granted (1) (2) | (4,129,419 | )
Nonvested equity stock and stock units forfeited (1) | 3,466,802 |
Total available for grant as of June 30, 2018 | 10,032,433 |
_________________________________________
(1) | For purposes of determining the number of shares available for grant under the 2015 Plan against the maximum number of shares authorized, each share of restricted stock granted reduces the number of shares available for grant by 1.5 shares and each share of restricted stock forfeited increases shares available for grant by 1.5 shares.
(2) | Amount includes 525,965 shares that have been reserved for potential future issuance related to certain performance unit awards granted in the first quarter of 2018 and discussed under the section titled "Nonvested Equity Stock and Stock Units" below.
Schedule of stock option activity | The following table summarizes stock option activity under the 2006 Plan and 2015 Plan for the six months ended June 30, 2018 and information regarding stock options outstanding, exercisable, and vested and expected to vest as of June 30, 2018.
| Options Outstanding |  |  |  |
| Number of Shares |  | Weighted Average Exercise Price Per Share |  | Weighted Average Remaining Contractual Term (years) |  | Aggregate Intrinsic Value
| (In thousands, except per share amounts)
Outstanding as of December 31, 2017 | 4,310,361 |  |  | $ | 9.78 |  |  |  |  |  |
Options granted | 621,479 |  |  | $ | 12.93 |  |  |  |  |  |
Options exercised | (336,004 | ) |  | $ | 7.48 |  |  |  |  |  |
Options forfeited | (765,382 | ) |  | $ | 13.60 |  |  |  |  |  |
Outstanding as of June 30, 2018 | 3,830,454 |  |  | $ | 9.73 |  |  | 3.73 |  | $ | 12,741 |
Vested or expected to vest at June 30, 2018 | 3,789,195 |  |  | $ | 9.70 |  |  | 3.67 |  | $ | 12,738 |
Options exercisable at June 30, 2018 | 3,109,152 |  |  | $ | 9.00 |  |  | 2.50 |  | $ | 12,659 |
Weighted-average assumptions for Stock Option Plans | The following table presents the weighted-average assumptions used to estimate the fair value of stock options granted that contain only service conditions in the periods presented.
| Stock Option Plan
| Three Months Ended |  | Six Months Ended
| June 30, |  | June 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
Stock Option Plan |  |  |  |  |  |  |  |  |  |  |
Expected stock price volatility | 24 | % |  | 32 | % |  | 24% - 29% |  |  | 32 | %
Risk free interest rate | 2.7 | % |  | 1.8 | % |  | 2.6% - 2.7% |  |  | 1.8% - 1.9% |
Expected term (in years) | 5.8 |  |  | 5.3 |  |  | 5.8 |  |  | 5.3 - 5.4 |
Weighted-average fair value of stock options granted to employees | $ | 3.94 |  |  | $ | 3.84 |  |  | $ | 4.21 |  |  | $ | 4.12 |
Schedule of nonvested equity stock and stock units activity | The following table reflects the activity related to nonvested equity stock and stock units for the six months ended June 30, 2018:
Nonvested Equity Stock and Stock Units |  | Shares |  | Weighted- Average Grant-Date Fair Value
Nonvested at December 31, 2017 |  | 5,861,349 |  |  | $ | 12.68 |
Granted |  | 2,402,303 |  |  | $ | 13.08 |
Vested |  | (1,127,011 | ) |  | $ | 11.99 |
Forfeited |  | (1,698,558 | ) |  | $ | 12.88 |
Nonvested at June 30, 2018 |  | 5,438,083 |  |  | $ | 12.95 |
Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block] |
| Employee Stock Purchase Plan
| Six Months Ended
| June 30,
| 2018 |  | 2017
Employee Stock Purchase Plan |  |  |  |  |
Expected stock price volatility | 27 | % |  | 27 | %
Risk free interest rate | 2.05 | % |  | 0.98 | %
Expected term (in years) | 0.5 |  |  | 0.5 |
Weighted-average fair value of purchase rights granted under the purchase plan | $ | 3.14 |  |  | $ | 2.87 |

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